From: John Groves <John@Groves.net>
* These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
* fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
newly stored as dev_dax->virt_addr by dev_dax_probe().
* The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
for read/write (dax_iomap_rw())
* fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
tested yet. I'm looking for suggestions as to how to test those.
* dax-private.h: add dev_dax->cached_size, which fsdev needs to
remember. The dev_dax size cannot change while a driver is bound
(dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
at probe time allows fsdev's direct_access path can use it without
acquiring dax_dev_rwsem (which isn't exported anyway).
Signed-off-by: John Groves <john@groves.net>
---
drivers/dax/dax-private.h | 1 +
drivers/dax/fsdev.c | 80 +++++++++++++++++++++++++++++++++++++++
2 files changed, 81 insertions(+)
diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
index 1bb1631af485..fbd8348cc71c 100644
--- a/drivers/dax/dax-private.h
+++ b/drivers/dax/dax-private.h
@@ -85,6 +85,7 @@ struct dev_dax {
struct dax_region *region;
struct dax_device *dax_dev;
void *virt_addr;
+ u64 cached_size;
unsigned int align;
int target_node;
bool dyn_id;
diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index c5c660b193e5..9e2f83aa2584 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -27,6 +27,81 @@
* - No mmap support - all access is through fs-dax/iomap
*/
+static void fsdev_write_dax(void *pmem_addr, struct page *page,
+ unsigned int off, unsigned int len)
+{
+ while (len) {
+ void *mem = kmap_local_page(page);
+ unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
+
+ memcpy_flushcache(pmem_addr, mem + off, chunk);
+ kunmap_local(mem);
+ len -= chunk;
+ off = 0;
+ page++;
+ pmem_addr += chunk;
+ }
+}
+
+static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
+ long nr_pages, enum dax_access_mode mode, void **kaddr,
+ unsigned long *pfn)
+{
+ struct dev_dax *dev_dax = dax_get_private(dax_dev);
+ size_t size = nr_pages << PAGE_SHIFT;
+ size_t offset = pgoff << PAGE_SHIFT;
+ void *virt_addr = dev_dax->virt_addr + offset;
+ phys_addr_t phys;
+ unsigned long local_pfn;
+
+ WARN_ON(!dev_dax->virt_addr);
+
+ phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
+
+ if (kaddr)
+ *kaddr = virt_addr;
+
+ local_pfn = PHYS_PFN(phys);
+ if (pfn)
+ *pfn = local_pfn;
+
+ /*
+ * Use cached_size which was computed at probe time. The size cannot
+ * change while the driver is bound (resize returns -EBUSY).
+ */
+ return PHYS_PFN(min_t(size_t, size, dev_dax->cached_size - offset));
+}
+
+static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
+ pgoff_t pgoff, size_t nr_pages)
+{
+ void *kaddr;
+
+ WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
+ __fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
+ fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
+ return 0;
+}
+
+static long fsdev_dax_direct_access(struct dax_device *dax_dev,
+ pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
+ void **kaddr, unsigned long *pfn)
+{
+ return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
+ kaddr, pfn);
+}
+
+static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
+ void *addr, size_t bytes, struct iov_iter *i)
+{
+ return _copy_from_iter_flushcache(addr, bytes, i);
+}
+
+static const struct dax_operations dev_dax_ops = {
+ .direct_access = fsdev_dax_direct_access,
+ .zero_page_range = fsdev_dax_zero_page_range,
+ .recovery_write = fsdev_dax_recovery_write,
+};
static void fsdev_cdev_del(void *cdev)
{
@@ -197,6 +272,11 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
}
}
+ /* Cache size now; it cannot change while driver is bound */
+ dev_dax->cached_size = 0;
+ for (i = 0; i < dev_dax->nr_range; i++)
+ dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
+
/*
* FS-DAX compatible mode: Use MEMORY_DEVICE_FS_DAX type and
* do NOT set vmemmap_shift. This leaves folios at order-0,
--
2.49.0On Wed, 7 Jan 2026 09:33:13 -0600
John Groves <John@Groves.net> wrote:
> From: John Groves <John@Groves.net>
>
Hi John
The description should generally make sense without the title.
Sometimes that means more or less repeating the title.
A few other things inline.
> * These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> * fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
> newly stored as dev_dax->virt_addr by dev_dax_probe().
> * The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
> for read/write (dax_iomap_rw())
> * fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
> tested yet. I'm looking for suggestions as to how to test those.
> * dax-private.h: add dev_dax->cached_size, which fsdev needs to
> remember. The dev_dax size cannot change while a driver is bound
> (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
> at probe time allows fsdev's direct_access path can use it without
> acquiring dax_dev_rwsem (which isn't exported anyway).
>
> Signed-off-by: John Groves <john@groves.net>
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index c5c660b193e5..9e2f83aa2584 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c
> @@ -27,6 +27,81 @@
> * - No mmap support - all access is through fs-dax/iomap
> */
>
> +static void fsdev_write_dax(void *pmem_addr, struct page *page,
> + unsigned int off, unsigned int len)
> +{
> + while (len) {
> + void *mem = kmap_local_page(page);
I guess it's pretty simple, but do we care about HIGHMEM for this
new feature? Maybe it's just easier to support it than argue about it however ;)
> + unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
> +
> + memcpy_flushcache(pmem_addr, mem + off, chunk);
> + kunmap_local(mem);
> + len -= chunk;
> + off = 0;
> + page++;
> + pmem_addr += chunk;
> + }
> +}
> +
> +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> + long nr_pages, enum dax_access_mode mode, void **kaddr,
> + unsigned long *pfn)
> +{
> + struct dev_dax *dev_dax = dax_get_private(dax_dev);
> + size_t size = nr_pages << PAGE_SHIFT;
> + size_t offset = pgoff << PAGE_SHIFT;
> + void *virt_addr = dev_dax->virt_addr + offset;
> + phys_addr_t phys;
> + unsigned long local_pfn;
> +
> + WARN_ON(!dev_dax->virt_addr);
> +
> + phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
Use size given you already computed it.
> +
> + if (kaddr)
> + *kaddr = virt_addr;
> +
> + local_pfn = PHYS_PFN(phys);
> + if (pfn)
> + *pfn = local_pfn;
> +
> + /*
> + * Use cached_size which was computed at probe time. The size cannot
> + * change while the driver is bound (resize returns -EBUSY).
> + */
> + return PHYS_PFN(min_t(size_t, size, dev_dax->cached_size - offset));
Is the min_t() needed? min() is pretty good at picking right types these days.
> +}
> +
> +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> + pgoff_t pgoff, size_t nr_pages)
> +{
> + void *kaddr;
> +
> + WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> + __fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> + fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> + return 0;
> +}
> +
> +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> + pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
> + void **kaddr, unsigned long *pfn)
> +{
> + return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> + kaddr, pfn);
Alignment in this file is a bit random, but I'd at least align this one
after the (
> +}
On 26/01/08 11:50AM, Jonathan Cameron wrote:
> On Wed, 7 Jan 2026 09:33:13 -0600
> John Groves <John@Groves.net> wrote:
>
> > From: John Groves <John@Groves.net>
> >
> Hi John
>
> The description should generally make sense without the title.
> Sometimes that means more or less repeating the title.
>
> A few other things inline.
Will do
>
> > * These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> > * fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
> > newly stored as dev_dax->virt_addr by dev_dax_probe().
> > * The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
> > for read/write (dax_iomap_rw())
> > * fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
> > tested yet. I'm looking for suggestions as to how to test those.
> > * dax-private.h: add dev_dax->cached_size, which fsdev needs to
> > remember. The dev_dax size cannot change while a driver is bound
> > (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
> > at probe time allows fsdev's direct_access path can use it without
> > acquiring dax_dev_rwsem (which isn't exported anyway).
> >
> > Signed-off-by: John Groves <john@groves.net>
>
> > diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> > index c5c660b193e5..9e2f83aa2584 100644
> > --- a/drivers/dax/fsdev.c
> > +++ b/drivers/dax/fsdev.c
> > @@ -27,6 +27,81 @@
> > * - No mmap support - all access is through fs-dax/iomap
> > */
> >
> > +static void fsdev_write_dax(void *pmem_addr, struct page *page,
> > + unsigned int off, unsigned int len)
> > +{
> > + while (len) {
> > + void *mem = kmap_local_page(page);
>
> I guess it's pretty simple, but do we care about HIGHMEM for this
> new feature? Maybe it's just easier to support it than argue about it however ;)
I think this compiles to zero overhead, and is an established pattern -
but I'm ok following a consensus elsewhere...
>
> > + unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
> > +
> > + memcpy_flushcache(pmem_addr, mem + off, chunk);
> > + kunmap_local(mem);
> > + len -= chunk;
> > + off = 0;
> > + page++;
> > + pmem_addr += chunk;
> > + }
> > +}
> > +
> > +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> > + long nr_pages, enum dax_access_mode mode, void **kaddr,
> > + unsigned long *pfn)
> > +{
> > + struct dev_dax *dev_dax = dax_get_private(dax_dev);
> > + size_t size = nr_pages << PAGE_SHIFT;
> > + size_t offset = pgoff << PAGE_SHIFT;
> > + void *virt_addr = dev_dax->virt_addr + offset;
> > + phys_addr_t phys;
> > + unsigned long local_pfn;
> > +
> > + WARN_ON(!dev_dax->virt_addr);
> > +
> > + phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
>
> Use size given you already computed it.
Not sure I follow. nr_pages is the size of the access or fault, not the size
of the device.
>
> > +
> > + if (kaddr)
> > + *kaddr = virt_addr;
> > +
> > + local_pfn = PHYS_PFN(phys);
> > + if (pfn)
> > + *pfn = local_pfn;
> > +
> > + /*
> > + * Use cached_size which was computed at probe time. The size cannot
> > + * change while the driver is bound (resize returns -EBUSY).
> > + */
> > + return PHYS_PFN(min_t(size_t, size, dev_dax->cached_size - offset));
>
> Is the min_t() needed? min() is pretty good at picking right types these days.
Changed to min()
>
> > +}
> > +
> > +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> > + pgoff_t pgoff, size_t nr_pages)
> > +{
> > + void *kaddr;
> > +
> > + WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> > + __fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> > + fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> > + return 0;
> > +}
> > +
> > +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> > + pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
> > + void **kaddr, unsigned long *pfn)
> > +{
> > + return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> > + kaddr, pfn);
>
> Alignment in this file is a bit random, but I'd at least align this one
> after the (
Done, thanks!
John
On Thu, 8 Jan 2026 09:59:08 -0600
John Groves <John@groves.net> wrote:
> On 26/01/08 11:50AM, Jonathan Cameron wrote:
> > On Wed, 7 Jan 2026 09:33:13 -0600
> > John Groves <John@Groves.net> wrote:
> >
> > > From: John Groves <John@Groves.net>
> > >
> > Hi John
> >
> > The description should generally make sense without the title.
> > Sometimes that means more or less repeating the title.
> >
> > A few other things inline.
>
> Will do
>
> >
> > > * These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> > > * fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
> > > newly stored as dev_dax->virt_addr by dev_dax_probe().
> > > * The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
> > > for read/write (dax_iomap_rw())
> > > * fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
> > > tested yet. I'm looking for suggestions as to how to test those.
> > > * dax-private.h: add dev_dax->cached_size, which fsdev needs to
> > > remember. The dev_dax size cannot change while a driver is bound
> > > (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
> > > at probe time allows fsdev's direct_access path can use it without
> > > acquiring dax_dev_rwsem (which isn't exported anyway).
> > >
> > > Signed-off-by: John Groves <john@groves.net>
> >
> > > diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> > > index c5c660b193e5..9e2f83aa2584 100644
> > > --- a/drivers/dax/fsdev.c
> > > +++ b/drivers/dax/fsdev.c
> > > @@ -27,6 +27,81 @@
> > > * - No mmap support - all access is through fs-dax/iomap
> > > */
> > >
> > > +static void fsdev_write_dax(void *pmem_addr, struct page *page,
> > > + unsigned int off, unsigned int len)
> > > +{
> > > + while (len) {
> > > + void *mem = kmap_local_page(page);
> >
> > I guess it's pretty simple, but do we care about HIGHMEM for this
> > new feature? Maybe it's just easier to support it than argue about it however ;)
>
> I think this compiles to zero overhead, and is an established pattern -
> but I'm ok following a consensus elsewhere...
That's fair, probably just keep it.
> > > +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> > > + long nr_pages, enum dax_access_mode mode, void **kaddr,
> > > + unsigned long *pfn)
> > > +{
> > > + struct dev_dax *dev_dax = dax_get_private(dax_dev);
> > > + size_t size = nr_pages << PAGE_SHIFT;
> > > + size_t offset = pgoff << PAGE_SHIFT;
> > > + void *virt_addr = dev_dax->virt_addr + offset;
> > > + phys_addr_t phys;
> > > + unsigned long local_pfn;
> > > +
> > > + WARN_ON(!dev_dax->virt_addr);
> > > +
> > > + phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
> >
> > Use size given you already computed it.
>
> Not sure I follow. nr_pages is the size of the access or fault, not the size
> of the device.
Just above:
size_t size = nr_pages << PAGE_SHIFT;
Jonathan
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