This option controls will mmap the memory backend file with MAP_SYNC flag,
which can ensure filesystem metadata consistent even after a system crash
or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
file on ext4/xfs file system mounted with '-o dax').

It can take one of following values:
 - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
        'share=off' or 'pmem!=on', QEMU will not pass this flags to
	mmap(2)
 - off: default, never pass MAP_SYNC to mmap(2)

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
---
 backends/hostmem-file.c   | 28 ++++++++++++++++++++++++++++
 docs/nvdimm.txt           | 23 ++++++++++++++++++++++-
 exec.c                    |  2 +-
 include/exec/memory.h     |  4 ++++
 include/exec/ram_addr.h   |  1 +
 include/qemu/mmap-alloc.h |  1 +
 qemu-options.hx           | 19 ++++++++++++++++++-
 util/mmap-alloc.c         |  4 ++--
 8 files changed, 77 insertions(+), 5 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 0dd7a90..3d39032 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -36,6 +36,7 @@ struct HostMemoryBackendFile {
     uint64_t align;
     bool discard_data;
     bool is_pmem;
+    bool sync;
 };
 
 static void
@@ -62,6 +63,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                  path,
                                  backend->size, fb->align,
                                  (backend->share ? RAM_SHARED : 0) |
+                                 (fb->sync ? RAM_SYNC : 0) |
                                  (fb->is_pmem ? RAM_PMEM : 0),
                                  fb->mem_path, errp);
         g_free(path);
@@ -136,6 +138,29 @@ static void file_memory_backend_set_align(Object *o, Visitor *v,
     error_propagate(errp, local_err);
 }
 
+static bool file_memory_backend_get_sync(Object *o, Error **errp)
+{
+    return MEMORY_BACKEND_FILE(o)->sync;
+}
+
+static void file_memory_backend_set_sync(
+    Object *obj, bool value, Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(errp, "cannot change property sync of %s",
+                   object_get_typename(obj));
+        goto out;
+    }
+
+    fb->sync = value;
+
+ out:
+    return;
+}
+
 static bool file_memory_backend_get_pmem(Object *o, Error **errp)
 {
     return MEMORY_BACKEND_FILE(o)->is_pmem;
@@ -203,6 +228,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
     object_class_property_add_bool(oc, "pmem",
         file_memory_backend_get_pmem, file_memory_backend_set_pmem,
         &error_abort);
+    object_class_property_add_bool(oc, "sync",
+        file_memory_backend_get_sync, file_memory_backend_set_sync,
+        &error_abort);
 }
 
 static void file_backend_instance_finalize(Object *o)
diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index 5f158a6..30db458 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -142,11 +142,32 @@ backend of vNVDIMM:
 Guest Data Persistence
 ----------------------
 
+vNVDIMM is designed and implemented to guarantee the guest data
+persistence on the backends even on the host crash and power
+failures. However, there are still some requirements and limitations
+as explained below.
+
 Though QEMU supports multiple types of vNVDIMM backends on Linux,
-currently the only one that can guarantee the guest write persistence
+if MAP_SYNC is not supported by the host kernel and the backends,
+the only backend that can guarantee the guest write persistence
 is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
 which all guest access do not involve any host-side kernel cache.
 
+mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
+systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
+filesystem metadata consistent even after a system crash or power
+failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
+also requires:
+
+ - the backend is a file supporting DAX, e.g., a file on an ext4 or
+   xfs file system mounted with '-o dax',
+
+ - 'sync' option of memory-backend-file is on, and
+
+ - 'share' option of memory-backend-file is 'on'.
+
+ - 'pmem' option of memory-backend-file is 'on'
+
 When using other types of backends, it's suggested to set 'unarmed'
 option of '-device nvdimm' to 'on', which sets the unarmed flag of the
 guest NVDIMM region mapping structure.  This unarmed flag indicates
diff --git a/exec.c b/exec.c
index e92a7da..dc4d180 100644
--- a/exec.c
+++ b/exec.c
@@ -2241,7 +2241,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
     int64_t file_size;
 
     /* Just support these ram flags by now. */
-    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM)) == 0);
+    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_SYNC)) == 0);
 
     if (xen_enabled()) {
         error_setg(errp, "-mem-path not supported with Xen");
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 6e30c23..9297b1c 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -26,6 +26,7 @@
 #include "qom/object.h"
 #include "qemu/rcu.h"
 #include "hw/qdev-core.h"
+#include "qapi/error.h"
 
 #define RAM_ADDR_INVALID (~(ram_addr_t)0)
 
@@ -136,6 +137,9 @@ typedef unsigned QEMU_BITWISE QemuMmapFlags;
 /* RAM is a persistent kind memory */
 #define RAM_PMEM ((QEMU_FORCE QemuMmapFlags) (1 << 5))
 
+/* RAM can be mmap by a MAP_SYNC flag */
+#define RAM_SYNC ((QEMU_FORCE QemuMmapFlags) (1 << 6))
+
 static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
                                        IOMMUNotifierFlag flags,
                                        hwaddr start, hwaddr end,
diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 9ecd911..d239ce7 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -87,6 +87,7 @@ long qemu_getrampagesize(void);
  *              or bit-or of following values
  *              - RAM_SHARED: mmap the backing file or device with MAP_SHARED
  *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
+ *              - RAM_SYNC:   mmap with MAP_SYNC flag
  *              Other bits are ignored.
  *  @mem_path or @fd: specify the backing file or device
  *  @errp: pointer to Error*, to store an error if it happens
diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
index 6fe6ed4..1755a8b 100644
--- a/include/qemu/mmap-alloc.h
+++ b/include/qemu/mmap-alloc.h
@@ -18,6 +18,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
  *  @flags: specifies additional properties of the mapping, which can be one or
  *          bit-or of following values
  *          - RAM_SHARED: mmap with MAP_SHARED flag
+ *          - RAM_SYNC:   mmap with MAP_SYNC flag
  *          Other bits are ignored.
  *
  * Return:
diff --git a/qemu-options.hx b/qemu-options.hx
index 08f8516..0f51d08 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3928,7 +3928,7 @@ property must be set.  These objects are placed in the
 
 @table @option
 
-@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align}
+@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align},sync=@var{on|off}
 
 Creates a memory file backend object, which can be used to back
 the guest RAM with huge pages.
@@ -4003,6 +4003,23 @@ If @option{pmem} is set to 'on', QEMU will take necessary operations to
 guarantee the persistence of its own writes to @option{mem-path}
 (e.g. in vNVDIMM label emulation and live migration).
 
+The @option{sync} option specifies whether QEMU mmap(2) @option{mem-path}
+with MAP_SYNC flag, which can ensure the file metadata is in sync to
+@option{mem-path} even on the host crash and power failures. MAP_SYNC
+requires supports from both the host kernel (since Linux kernel 4.15)
+and @option{mem-path} (only files supporting DAX). It can take one of
+following values:
+
+@table @option
+@item @var{on}
+try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
+@option{share}=@var{off}, @option{pmem}=@var{off} QEMU will not pass
+this flags to kernel.
+
+@item @var{off} (default)
+never pass MAP_SYNC to mmap(2)
+@end table
+
 @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}
 
 Creates a memory backend object, which can be used to back the guest RAM.
diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index a9d5e56..33a7639 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
     void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
 #endif
     bool shared = flags & RAM_SHARED;
-    bool is_pmem = flags & RAM_PMEM;
+    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);
     int mmap_xflags = 0;
     size_t offset;
     void *ptr1;
@@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
     assert(is_power_of_2(align));
     /* Always align to host page size */
     assert(align >= getpagesize());
-    if (shared && is_pmem) {
+    if (shared && is_pmemsync) {
         mmap_xflags |= MAP_SYNC;
     }
 
-- 
2.7.4

On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote:
> This option controls will mmap the memory backend file with MAP_SYNC flag,
> which can ensure filesystem metadata consistent even after a system crash
> or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
> kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
> file on ext4/xfs file system mounted with '-o dax').
> 
> It can take one of following values:
>  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
>         'share=off' or 'pmem!=on', QEMU will not pass this flags to
> 	mmap(2)
>  - off: default, never pass MAP_SYNC to mmap(2)
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> ---
[...]
> +vNVDIMM is designed and implemented to guarantee the guest data
> +persistence on the backends even on the host crash and power
> +failures. However, there are still some requirements and limitations
> +as explained below.
> +
>  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> -currently the only one that can guarantee the guest write persistence
> +if MAP_SYNC is not supported by the host kernel and the backends,
> +the only backend that can guarantee the guest write persistence
>  is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
>  which all guest access do not involve any host-side kernel cache.
>  
> +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
> +filesystem metadata consistent even after a system crash or power
> +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> +also requires:
> +
> + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> +   xfs file system mounted with '-o dax',
> +
> + - 'sync' option of memory-backend-file is on, and
> +
> + - 'share' option of memory-backend-file is 'on'.
> +
> + - 'pmem' option of memory-backend-file is 'on'

I miss one piece of information here: are there any negative
side-effects of enabling MAP_SYNC on a pmem=on backend?  Could it
affect performance?  If it has no negative effects, why don't we
try to always enable it whenever possible?


> +
>  When using other types of backends, it's suggested to set 'unarmed'
>  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
>  guest NVDIMM region mapping structure.  This unarmed flag indicates
[...]
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index a9d5e56..33a7639 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>  #endif
>      bool shared = flags & RAM_SHARED;
> -    bool is_pmem = flags & RAM_PMEM;
> +    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);

You seem to be reverting what you did on patch 3/5.  In patch
3/5, you were setting MAP_SYNC automatically on all pmem=on
backends.  Now, you are only setting MAP_SYNC only if sync=on is
set explicitly.

I don't know which behavior is better (see question above), but
it's better to start with the right behavior in the first place.

Also, I don't think we should clear MAP_SYNC silently if sync=on
was explicitly requested in the command-line.  If sync=on was
set, we should do exactly as told, and require MAP_SYNC.  If we
still want to support use cases where MAP_SYNC is desired but
optional (do we?), we can make 'sync' a OnOffAuto option.


>      int mmap_xflags = 0;
>      size_t offset;
>      void *ptr1;
> @@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      assert(is_power_of_2(align));
>      /* Always align to host page size */
>      assert(align >= getpagesize());
> -    if (shared && is_pmem) {
> +    if (shared && is_pmemsync) {
>          mmap_xflags |= MAP_SYNC;
>      }
>  
> -- 
> 2.7.4
> 
> 

-- 
Eduardo

On 2019-01-14 at 17:39:38 -0200, Eduardo Habkost wrote:
> On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote:
> > This option controls will mmap the memory backend file with MAP_SYNC flag,
> > which can ensure filesystem metadata consistent even after a system crash
> > or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
> > kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
> > file on ext4/xfs file system mounted with '-o dax').
> > 
> > It can take one of following values:
> >  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
> >         'share=off' or 'pmem!=on', QEMU will not pass this flags to
> > 	mmap(2)
> >  - off: default, never pass MAP_SYNC to mmap(2)
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > ---
> [...]
> > +vNVDIMM is designed and implemented to guarantee the guest data
> > +persistence on the backends even on the host crash and power
> > +failures. However, there are still some requirements and limitations
> > +as explained below.
> > +
> >  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> > -currently the only one that can guarantee the guest write persistence
> > +if MAP_SYNC is not supported by the host kernel and the backends,
> > +the only backend that can guarantee the guest write persistence
> >  is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
> >  which all guest access do not involve any host-side kernel cache.
> >  
> > +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> > +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
> > +filesystem metadata consistent even after a system crash or power
> > +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> > +also requires:
> > +
> > + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> > +   xfs file system mounted with '-o dax',
> > +
> > + - 'sync' option of memory-backend-file is on, and
> > +
> > + - 'share' option of memory-backend-file is 'on'.
> > +
> > + - 'pmem' option of memory-backend-file is 'on'
> 
> I miss one piece of information here: are there any negative
> side-effects of enabling MAP_SYNC on a pmem=on backend?  Could it
> affect performance?  If it has no negative effects, why don't we
> try to always enable it whenever possible?
> 
> 
> > +
> >  When using other types of backends, it's suggested to set 'unarmed'
> >  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
> >  guest NVDIMM region mapping structure.  This unarmed flag indicates
> [...]
> > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> > index a9d5e56..33a7639 100644
> > --- a/util/mmap-alloc.c
> > +++ b/util/mmap-alloc.c
> > @@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
> >      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> >  #endif
> >      bool shared = flags & RAM_SHARED;
> > -    bool is_pmem = flags & RAM_PMEM;
> > +    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);
> 
> You seem to be reverting what you did on patch 3/5.  In patch
> 3/5, you were setting MAP_SYNC automatically on all pmem=on
> backends.  Now, you are only setting MAP_SYNC only if sync=on is
> set explicitly.
> 
> I don't know which behavior is better (see question above), but
> it's better to start with the right behavior in the first place.
> 
> Also, I don't think we should clear MAP_SYNC silently if sync=on
> was explicitly requested in the command-line.  If sync=on was
> set, we should do exactly as told, and require MAP_SYNC.  If we
> still want to support use cases where MAP_SYNC is desired but
> optional (do we?), we can make 'sync' a OnOffAuto option.
Actually, I did this on previous version.
see https://patchwork.kernel.org/patch/10725671/ 

Michael said that we should limit that option as it is only valided
on a dax aware file system, to avoid the potencial performance issues
we set it off by-defualt, and let a well-know user decides they wanna
performance or stability.
> 
> 
> >      int mmap_xflags = 0;
> >      size_t offset;
> >      void *ptr1;
> > @@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
> >      assert(is_power_of_2(align));
> >      /* Always align to host page size */
> >      assert(align >= getpagesize());
> > -    if (shared && is_pmem) {
> > +    if (shared && is_pmemsync) {
> >          mmap_xflags |= MAP_SYNC;
> >      }
> >  
> > -- 
> > 2.7.4
> > 
> > 
> 
> -- 
> Eduardo

On Tue, Jan 15, 2019 at 11:13:35AM +0800, Yi Zhang wrote:
> On 2019-01-14 at 17:39:38 -0200, Eduardo Habkost wrote:
> > On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote:
> > > This option controls will mmap the memory backend file with MAP_SYNC flag,
> > > which can ensure filesystem metadata consistent even after a system crash
> > > or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
> > > kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
> > > file on ext4/xfs file system mounted with '-o dax').
> > > 
> > > It can take one of following values:
> > >  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
> > >         'share=off' or 'pmem!=on', QEMU will not pass this flags to
> > > 	mmap(2)
> > >  - off: default, never pass MAP_SYNC to mmap(2)
> > > 
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> > > ---
> > [...]
> > > +vNVDIMM is designed and implemented to guarantee the guest data
> > > +persistence on the backends even on the host crash and power
> > > +failures. However, there are still some requirements and limitations
> > > +as explained below.
> > > +
> > >  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> > > -currently the only one that can guarantee the guest write persistence
> > > +if MAP_SYNC is not supported by the host kernel and the backends,
> > > +the only backend that can guarantee the guest write persistence
> > >  is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
> > >  which all guest access do not involve any host-side kernel cache.
> > >  
> > > +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> > > +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
> > > +filesystem metadata consistent even after a system crash or power
> > > +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> > > +also requires:
> > > +
> > > + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> > > +   xfs file system mounted with '-o dax',
> > > +
> > > + - 'sync' option of memory-backend-file is on, and
> > > +
> > > + - 'share' option of memory-backend-file is 'on'.
> > > +
> > > + - 'pmem' option of memory-backend-file is 'on'
> > 
> > I miss one piece of information here: are there any negative
> > side-effects of enabling MAP_SYNC on a pmem=on backend?  Could it
> > affect performance?  If it has no negative effects, why don't we
> > try to always enable it whenever possible?
> > 
> > 
> > > +
> > >  When using other types of backends, it's suggested to set 'unarmed'
> > >  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
> > >  guest NVDIMM region mapping structure.  This unarmed flag indicates
> > [...]
> > > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> > > index a9d5e56..33a7639 100644
> > > --- a/util/mmap-alloc.c
> > > +++ b/util/mmap-alloc.c
> > > @@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
> > >      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> > >  #endif
> > >      bool shared = flags & RAM_SHARED;
> > > -    bool is_pmem = flags & RAM_PMEM;
> > > +    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);
> > 
> > You seem to be reverting what you did on patch 3/5.  In patch
> > 3/5, you were setting MAP_SYNC automatically on all pmem=on
> > backends.  Now, you are only setting MAP_SYNC only if sync=on is
> > set explicitly.
> > 
> > I don't know which behavior is better (see question above), but
> > it's better to start with the right behavior in the first place.
> > 
> > Also, I don't think we should clear MAP_SYNC silently if sync=on
> > was explicitly requested in the command-line.  If sync=on was
> > set, we should do exactly as told, and require MAP_SYNC.  If we
> > still want to support use cases where MAP_SYNC is desired but
> > optional (do we?), we can make 'sync' a OnOffAuto option.
> Actually, I did this on previous version.
> see https://patchwork.kernel.org/patch/10725671/ 
> 
> Michael said that we should limit that option as it is only valided
> on a dax aware file system, to avoid the potencial performance issues
> we set it off by-defualt, and let a well-know user decides they wanna
> performance or stability.

However I am still unconvinced that the separate sync flag is helpful.
Why don't we set MAP_SYNC unconditionally when pmem is set?

It's a separate question what should happen on an old kernel. Maybe we
want a flag that says "fail unless persistence can be guaranteed".
Even then it's definitely not "sync".






> > 
> > 
> > >      int mmap_xflags = 0;
> > >      size_t offset;
> > >      void *ptr1;
> > > @@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
> > >      assert(is_power_of_2(align));
> > >      /* Always align to host page size */
> > >      assert(align >= getpagesize());
> > > -    if (shared && is_pmem) {
> > > +    if (shared && is_pmemsync) {
> > >          mmap_xflags |= MAP_SYNC;
> > >      }
> > >  
> > > -- 
> > > 2.7.4
> > > 
> > > 
> > 
> > -- 
> > Eduardo

On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote:
> This option controls will mmap the memory backend file with MAP_SYNC flag,
> which can ensure filesystem metadata consistent even after a system crash
> or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
> kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
> file on ext4/xfs file system mounted with '-o dax').
> 
> It can take one of following values:
>  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
>         'share=off' or 'pmem!=on', QEMU will not pass this flags to
> 	mmap(2)
>  - off: default, never pass MAP_SYNC to mmap(2)
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>


So we introduce all of the above complexity and then I am pretty sure go
on and teach management tools to just always, without exception, set
sync=on to avoid data corruption.

So how about we give up on a bit of flexibility, and just say
pmem=on forces MAP_SYNC?

OTOH if you really really want a fast memory then why set pmem=on at
all?

Or, if you have some data that shows how disabling synchronous
pagefaults helps performance a lot, maybe we should introduce
a "crash-unsafe" flag.



> ---
>  backends/hostmem-file.c   | 28 ++++++++++++++++++++++++++++
>  docs/nvdimm.txt           | 23 ++++++++++++++++++++++-
>  exec.c                    |  2 +-
>  include/exec/memory.h     |  4 ++++
>  include/exec/ram_addr.h   |  1 +
>  include/qemu/mmap-alloc.h |  1 +
>  qemu-options.hx           | 19 ++++++++++++++++++-
>  util/mmap-alloc.c         |  4 ++--
>  8 files changed, 77 insertions(+), 5 deletions(-)
> 
> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> index 0dd7a90..3d39032 100644
> --- a/backends/hostmem-file.c
> +++ b/backends/hostmem-file.c
> @@ -36,6 +36,7 @@ struct HostMemoryBackendFile {
>      uint64_t align;
>      bool discard_data;
>      bool is_pmem;
> +    bool sync;
>  };
>  
>  static void
> @@ -62,6 +63,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>                                   path,
>                                   backend->size, fb->align,
>                                   (backend->share ? RAM_SHARED : 0) |
> +                                 (fb->sync ? RAM_SYNC : 0) |
>                                   (fb->is_pmem ? RAM_PMEM : 0),
>                                   fb->mem_path, errp);
>          g_free(path);
> @@ -136,6 +138,29 @@ static void file_memory_backend_set_align(Object *o, Visitor *v,
>      error_propagate(errp, local_err);
>  }
>  
> +static bool file_memory_backend_get_sync(Object *o, Error **errp)
> +{
> +    return MEMORY_BACKEND_FILE(o)->sync;
> +}
> +
> +static void file_memory_backend_set_sync(
> +    Object *obj, bool value, Error **errp)
> +{
> +    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
> +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
> +
> +    if (host_memory_backend_mr_inited(backend)) {
> +        error_setg(errp, "cannot change property sync of %s",
> +                   object_get_typename(obj));
> +        goto out;
> +    }
> +
> +    fb->sync = value;
> +
> + out:
> +    return;
> +}
> +
>  static bool file_memory_backend_get_pmem(Object *o, Error **errp)
>  {
>      return MEMORY_BACKEND_FILE(o)->is_pmem;
> @@ -203,6 +228,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
>      object_class_property_add_bool(oc, "pmem",
>          file_memory_backend_get_pmem, file_memory_backend_set_pmem,
>          &error_abort);
> +    object_class_property_add_bool(oc, "sync",
> +        file_memory_backend_get_sync, file_memory_backend_set_sync,
> +        &error_abort);
>  }
>  
>  static void file_backend_instance_finalize(Object *o)
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index 5f158a6..30db458 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -142,11 +142,32 @@ backend of vNVDIMM:
>  Guest Data Persistence
>  ----------------------
>  
> +vNVDIMM is designed and implemented to guarantee the guest data
> +persistence on the backends even on the host crash and power
> +failures. However, there are still some requirements and limitations
> +as explained below.
> +
>  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> -currently the only one that can guarantee the guest write persistence
> +if MAP_SYNC is not supported by the host kernel and the backends,
> +the only backend that can guarantee the guest write persistence
>  is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
>  which all guest access do not involve any host-side kernel cache.
>  
> +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
> +filesystem metadata consistent even after a system crash or power
> +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> +also requires:
> +
> + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> +   xfs file system mounted with '-o dax',
> +
> + - 'sync' option of memory-backend-file is on, and
> +
> + - 'share' option of memory-backend-file is 'on'.
> +
> + - 'pmem' option of memory-backend-file is 'on'
> +
>  When using other types of backends, it's suggested to set 'unarmed'
>  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
>  guest NVDIMM region mapping structure.  This unarmed flag indicates
> diff --git a/exec.c b/exec.c
> index e92a7da..dc4d180 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2241,7 +2241,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>      int64_t file_size;
>  
>      /* Just support these ram flags by now. */
> -    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM)) == 0);
> +    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_SYNC)) == 0);
>  
>      if (xen_enabled()) {
>          error_setg(errp, "-mem-path not supported with Xen");
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 6e30c23..9297b1c 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -26,6 +26,7 @@
>  #include "qom/object.h"
>  #include "qemu/rcu.h"
>  #include "hw/qdev-core.h"
> +#include "qapi/error.h"
>  
>  #define RAM_ADDR_INVALID (~(ram_addr_t)0)
>  
> @@ -136,6 +137,9 @@ typedef unsigned QEMU_BITWISE QemuMmapFlags;
>  /* RAM is a persistent kind memory */
>  #define RAM_PMEM ((QEMU_FORCE QemuMmapFlags) (1 << 5))
>  
> +/* RAM can be mmap by a MAP_SYNC flag */
> +#define RAM_SYNC ((QEMU_FORCE QemuMmapFlags) (1 << 6))
> +
>  static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
>                                         IOMMUNotifierFlag flags,
>                                         hwaddr start, hwaddr end,
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index 9ecd911..d239ce7 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -87,6 +87,7 @@ long qemu_getrampagesize(void);
>   *              or bit-or of following values
>   *              - RAM_SHARED: mmap the backing file or device with MAP_SHARED
>   *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
> + *              - RAM_SYNC:   mmap with MAP_SYNC flag
>   *              Other bits are ignored.
>   *  @mem_path or @fd: specify the backing file or device
>   *  @errp: pointer to Error*, to store an error if it happens
> diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
> index 6fe6ed4..1755a8b 100644
> --- a/include/qemu/mmap-alloc.h
> +++ b/include/qemu/mmap-alloc.h
> @@ -18,6 +18,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
>   *  @flags: specifies additional properties of the mapping, which can be one or
>   *          bit-or of following values
>   *          - RAM_SHARED: mmap with MAP_SHARED flag
> + *          - RAM_SYNC:   mmap with MAP_SYNC flag
>   *          Other bits are ignored.
>   *
>   * Return:
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 08f8516..0f51d08 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3928,7 +3928,7 @@ property must be set.  These objects are placed in the
>  
>  @table @option
>  
> -@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align}
> +@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align},sync=@var{on|off}
>  
>  Creates a memory file backend object, which can be used to back
>  the guest RAM with huge pages.
> @@ -4003,6 +4003,23 @@ If @option{pmem} is set to 'on', QEMU will take necessary operations to
>  guarantee the persistence of its own writes to @option{mem-path}
>  (e.g. in vNVDIMM label emulation and live migration).
>  
> +The @option{sync} option specifies whether QEMU mmap(2) @option{mem-path}
> +with MAP_SYNC flag, which can ensure the file metadata is in sync to
> +@option{mem-path} even on the host crash and power failures. MAP_SYNC
> +requires supports from both the host kernel (since Linux kernel 4.15)
> +and @option{mem-path} (only files supporting DAX). It can take one of
> +following values:
> +
> +@table @option
> +@item @var{on}
> +try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
> +@option{share}=@var{off}, @option{pmem}=@var{off} QEMU will not pass
> +this flags to kernel.
> +
> +@item @var{off} (default)
> +never pass MAP_SYNC to mmap(2)
> +@end table
> +
>  @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}
>  
>  Creates a memory backend object, which can be used to back the guest RAM.
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index a9d5e56..33a7639 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>  #endif
>      bool shared = flags & RAM_SHARED;
> -    bool is_pmem = flags & RAM_PMEM;
> +    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);
>      int mmap_xflags = 0;
>      size_t offset;
>      void *ptr1;
> @@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      assert(is_power_of_2(align));
>      /* Always align to host page size */
>      assert(align >= getpagesize());
> -    if (shared && is_pmem) {
> +    if (shared && is_pmemsync) {
>          mmap_xflags |= MAP_SYNC;
>      }
>  
> -- 
> 2.7.4

On 2019-01-14 at 22:31:45 -0500, Michael S. Tsirkin wrote:
> On Wed, Jan 02, 2019 at 01:26:34PM +0800, Zhang Yi wrote:
> > This option controls will mmap the memory backend file with MAP_SYNC flag,
> > which can ensure filesystem metadata consistent even after a system crash
> > or power failure, if MAP_SYNC flag is supported by the host kernel(Linux
> > kernel 4.15 and later) and the backend is a file supporting DAX (e.g.,
> > file on ext4/xfs file system mounted with '-o dax').
> > 
> > It can take one of following values:
> >  - on:  try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
> >         'share=off' or 'pmem!=on', QEMU will not pass this flags to
> > 	mmap(2)
> >  - off: default, never pass MAP_SYNC to mmap(2)
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> 
> 
> So we introduce all of the above complexity and then I am pretty sure go
> on and teach management tools to just always, without exception, set
> sync=on to avoid data corruption.
> 
> So how about we give up on a bit of flexibility, and just say
> pmem=on forces MAP_SYNC?
> 
> OTOH if you really really want a fast memory then why set pmem=on at
> all?

Indeed, All my concern is that we do need to pass the sync to a type of
pmem which didn't backend on a dax aware file. Anyway, I will drop the
sync option, and let it on while we set pmem, Thanks your suggestion.
Michael.

> 
> Or, if you have some data that shows how disabling synchronous
> pagefaults helps performance a lot, maybe we should introduce
> a "crash-unsafe" flag.
> 
> 
> 
> > ---
> >  backends/hostmem-file.c   | 28 ++++++++++++++++++++++++++++
> >  docs/nvdimm.txt           | 23 ++++++++++++++++++++++-
> >  exec.c                    |  2 +-
> >  include/exec/memory.h     |  4 ++++
> >  include/exec/ram_addr.h   |  1 +
> >  include/qemu/mmap-alloc.h |  1 +
> >  qemu-options.hx           | 19 ++++++++++++++++++-
> >  util/mmap-alloc.c         |  4 ++--
> >  8 files changed, 77 insertions(+), 5 deletions(-)
> > 
> > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> > index 0dd7a90..3d39032 100644
> > --- a/backends/hostmem-file.c
> > +++ b/backends/hostmem-file.c
> > @@ -36,6 +36,7 @@ struct HostMemoryBackendFile {
> >      uint64_t align;
> >      bool discard_data;
> >      bool is_pmem;
> > +    bool sync;
> >  };
> >  
> >  static void
> > @@ -62,6 +63,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
> >                                   path,
> >                                   backend->size, fb->align,
> >                                   (backend->share ? RAM_SHARED : 0) |
> > +                                 (fb->sync ? RAM_SYNC : 0) |
> >                                   (fb->is_pmem ? RAM_PMEM : 0),
> >                                   fb->mem_path, errp);
> >          g_free(path);
> > @@ -136,6 +138,29 @@ static void file_memory_backend_set_align(Object *o, Visitor *v,
> >      error_propagate(errp, local_err);
> >  }
> >  
> > +static bool file_memory_backend_get_sync(Object *o, Error **errp)
> > +{
> > +    return MEMORY_BACKEND_FILE(o)->sync;
> > +}
> > +
> > +static void file_memory_backend_set_sync(
> > +    Object *obj, bool value, Error **errp)
> > +{
> > +    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
> > +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
> > +
> > +    if (host_memory_backend_mr_inited(backend)) {
> > +        error_setg(errp, "cannot change property sync of %s",
> > +                   object_get_typename(obj));
> > +        goto out;
> > +    }
> > +
> > +    fb->sync = value;
> > +
> > + out:
> > +    return;
> > +}
> > +
> >  static bool file_memory_backend_get_pmem(Object *o, Error **errp)
> >  {
> >      return MEMORY_BACKEND_FILE(o)->is_pmem;
> > @@ -203,6 +228,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
> >      object_class_property_add_bool(oc, "pmem",
> >          file_memory_backend_get_pmem, file_memory_backend_set_pmem,
> >          &error_abort);
> > +    object_class_property_add_bool(oc, "sync",
> > +        file_memory_backend_get_sync, file_memory_backend_set_sync,
> > +        &error_abort);
> >  }
> >  
> >  static void file_backend_instance_finalize(Object *o)
> > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> > index 5f158a6..30db458 100644
> > --- a/docs/nvdimm.txt
> > +++ b/docs/nvdimm.txt
> > @@ -142,11 +142,32 @@ backend of vNVDIMM:
> >  Guest Data Persistence
> >  ----------------------
> >  
> > +vNVDIMM is designed and implemented to guarantee the guest data
> > +persistence on the backends even on the host crash and power
> > +failures. However, there are still some requirements and limitations
> > +as explained below.
> > +
> >  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> > -currently the only one that can guarantee the guest write persistence
> > +if MAP_SYNC is not supported by the host kernel and the backends,
> > +the only backend that can guarantee the guest write persistence
> >  is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
> >  which all guest access do not involve any host-side kernel cache.
> >  
> > +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> > +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
> > +filesystem metadata consistent even after a system crash or power
> > +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> > +also requires:
> > +
> > + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> > +   xfs file system mounted with '-o dax',
> > +
> > + - 'sync' option of memory-backend-file is on, and
> > +
> > + - 'share' option of memory-backend-file is 'on'.
> > +
> > + - 'pmem' option of memory-backend-file is 'on'
> > +
> >  When using other types of backends, it's suggested to set 'unarmed'
> >  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
> >  guest NVDIMM region mapping structure.  This unarmed flag indicates
> > diff --git a/exec.c b/exec.c
> > index e92a7da..dc4d180 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -2241,7 +2241,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
> >      int64_t file_size;
> >  
> >      /* Just support these ram flags by now. */
> > -    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM)) == 0);
> > +    assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_SYNC)) == 0);
> >  
> >      if (xen_enabled()) {
> >          error_setg(errp, "-mem-path not supported with Xen");
> > diff --git a/include/exec/memory.h b/include/exec/memory.h
> > index 6e30c23..9297b1c 100644
> > --- a/include/exec/memory.h
> > +++ b/include/exec/memory.h
> > @@ -26,6 +26,7 @@
> >  #include "qom/object.h"
> >  #include "qemu/rcu.h"
> >  #include "hw/qdev-core.h"
> > +#include "qapi/error.h"
> >  
> >  #define RAM_ADDR_INVALID (~(ram_addr_t)0)
> >  
> > @@ -136,6 +137,9 @@ typedef unsigned QEMU_BITWISE QemuMmapFlags;
> >  /* RAM is a persistent kind memory */
> >  #define RAM_PMEM ((QEMU_FORCE QemuMmapFlags) (1 << 5))
> >  
> > +/* RAM can be mmap by a MAP_SYNC flag */
> > +#define RAM_SYNC ((QEMU_FORCE QemuMmapFlags) (1 << 6))
> > +
> >  static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
> >                                         IOMMUNotifierFlag flags,
> >                                         hwaddr start, hwaddr end,
> > diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> > index 9ecd911..d239ce7 100644
> > --- a/include/exec/ram_addr.h
> > +++ b/include/exec/ram_addr.h
> > @@ -87,6 +87,7 @@ long qemu_getrampagesize(void);
> >   *              or bit-or of following values
> >   *              - RAM_SHARED: mmap the backing file or device with MAP_SHARED
> >   *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
> > + *              - RAM_SYNC:   mmap with MAP_SYNC flag
> >   *              Other bits are ignored.
> >   *  @mem_path or @fd: specify the backing file or device
> >   *  @errp: pointer to Error*, to store an error if it happens
> > diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
> > index 6fe6ed4..1755a8b 100644
> > --- a/include/qemu/mmap-alloc.h
> > +++ b/include/qemu/mmap-alloc.h
> > @@ -18,6 +18,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
> >   *  @flags: specifies additional properties of the mapping, which can be one or
> >   *          bit-or of following values
> >   *          - RAM_SHARED: mmap with MAP_SHARED flag
> > + *          - RAM_SYNC:   mmap with MAP_SYNC flag
> >   *          Other bits are ignored.
> >   *
> >   * Return:
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 08f8516..0f51d08 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -3928,7 +3928,7 @@ property must be set.  These objects are placed in the
> >  
> >  @table @option
> >  
> > -@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align}
> > +@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},align=@var{align},sync=@var{on|off}
> >  
> >  Creates a memory file backend object, which can be used to back
> >  the guest RAM with huge pages.
> > @@ -4003,6 +4003,23 @@ If @option{pmem} is set to 'on', QEMU will take necessary operations to
> >  guarantee the persistence of its own writes to @option{mem-path}
> >  (e.g. in vNVDIMM label emulation and live migration).
> >  
> > +The @option{sync} option specifies whether QEMU mmap(2) @option{mem-path}
> > +with MAP_SYNC flag, which can ensure the file metadata is in sync to
> > +@option{mem-path} even on the host crash and power failures. MAP_SYNC
> > +requires supports from both the host kernel (since Linux kernel 4.15)
> > +and @option{mem-path} (only files supporting DAX). It can take one of
> > +following values:
> > +
> > +@table @option
> > +@item @var{on}
> > +try to pass MAP_SYNC to mmap(2); if MAP_SYNC is not supported or
> > +@option{share}=@var{off}, @option{pmem}=@var{off} QEMU will not pass
> > +this flags to kernel.
> > +
> > +@item @var{off} (default)
> > +never pass MAP_SYNC to mmap(2)
> > +@end table
> > +
> >  @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}
> >  
> >  Creates a memory backend object, which can be used to back the guest RAM.
> > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> > index a9d5e56..33a7639 100644
> > --- a/util/mmap-alloc.c
> > +++ b/util/mmap-alloc.c
> > @@ -99,7 +99,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
> >      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> >  #endif
> >      bool shared = flags & RAM_SHARED;
> > -    bool is_pmem = flags & RAM_PMEM;
> > +    bool is_pmemsync = (flags & RAM_PMEM) && (flags & RAM_SYNC);
> >      int mmap_xflags = 0;
> >      size_t offset;
> >      void *ptr1;
> > @@ -111,7 +111,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
> >      assert(is_power_of_2(align));
> >      /* Always align to host page size */
> >      assert(align >= getpagesize());
> > -    if (shared && is_pmem) {
> > +    if (shared && is_pmemsync) {
> >          mmap_xflags |= MAP_SYNC;
> >      }
> >  
> > -- 
> > 2.7.4