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From: Akihiko Odaki <akihiko.odaki@daynix.com>
Date: Thu, 09 Jan 2025 14:50:20 +0900
Subject: [PATCH v7 1/2] memory: Update inline documentation
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Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-ppc@nongnu.org,
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Do not refer to "memory region's reference count"
Reviewed-by: Peter Xu <peterx@redhat.com>
-------------------------------------------------
Now MemoryRegions do have their own reference counts, but they will not
be used when their owners are not themselves. However, the documentation
of memory_region_ref() says it adds "1 to a memory region's reference
count", which is confusing. Avoid referring to "memory region's
reference count" and just say: "Add a reference to a memory region".
Make a similar change to memory_region_unref() too.
Refer to docs/devel/memory.rst for "owner"
------------------------------------------
memory_region_ref() and memory_region_unref() used to have their own
descriptions of "owner", but they are somewhat out-of-date and
misleading.
In particular, they say "whenever memory regions are accessed outside
the BQL, they need to be preserved against hot-unplug", but protecting
against hot-unplug is not mandatory if it is known that they will never
be hot-unplugged. They also say "MemoryRegions actually do not have
their own reference count", but they actually do. They just will not be
used unless their owners are not themselves.
Refer to docs/devel/memory.rst as the single source of truth instead of
maintaining duplicate descriptions of "owner".
Clarify that owner may be missing
---------------------------------
A memory region may not have an owner, and memory_region_ref() and
memory_region_unref() do nothing for such.
memory: Clarify owner must not call memory_region_ref()
--------------------------------------------------------
The owner must not call this function as it results in a circular
reference.
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
---
include/exec/memory.h | 59 ++++++++++++++++++++++++-----------------------=
----
1 file changed, 28 insertions(+), 31 deletions(-)
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 9458e2801d50..ca247343f433 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -1210,7 +1210,7 @@ void memory_region_section_free_copy(MemoryRegionSect=
ion *s);
* memory_region_add_subregion() to add subregions.
*
* @mr: the #MemoryRegion to be initialized
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: used for debugging; not visible to the user or ABI
* @size: size of the region; any subregions beyond this size will be clip=
ped
*/
@@ -1220,29 +1220,26 @@ void memory_region_init(MemoryRegion *mr,
uint64_t size);
=20
/**
- * memory_region_ref: Add 1 to a memory region's reference count
+ * memory_region_ref: Add a reference to the owner of a memory region
*
- * Whenever memory regions are accessed outside the BQL, they need to be
- * preserved against hot-unplug. MemoryRegions actually do not have their
- * own reference count; they piggyback on a QOM object, their "owner".
- * This function adds a reference to the owner.
- *
- * All MemoryRegions must have an owner if they can disappear, even if the
- * device they belong to operates exclusively under the BQL. This is beca=
use
- * the region could be returned at any time by memory_region_find, and this
- * is usually under guest control.
+ * This function adds a reference to the owner of a memory region to keep =
the
+ * memory region alive. It does nothing if the owner is not present as a m=
emory
+ * region without owner will never die.
+ * For references internal to the owner, use object_ref() instead to avoid=
a
+ * circular reference.
+ * See docs/devel/memory.rst to know about owner.
*
* @mr: the #MemoryRegion
*/
void memory_region_ref(MemoryRegion *mr);
=20
/**
- * memory_region_unref: Remove 1 to a memory region's reference count
+ * memory_region_unref: Remove a reference to the memory region of the own=
er
*
- * Whenever memory regions are accessed outside the BQL, they need to be
- * preserved against hot-unplug. MemoryRegions actually do not have their
- * own reference count; they piggyback on a QOM object, their "owner".
- * This function removes a reference to the owner and possibly destroys it.
+ * This function removes a reference to the owner of a memory region and
+ * possibly destroys the owner along with the memory region. It does nothi=
ng if
+ * the owner is not present.
+ * See docs/devel/memory.rst to know about owner.
*
* @mr: the #MemoryRegion
*/
@@ -1255,7 +1252,7 @@ void memory_region_unref(MemoryRegion *mr);
* if @size is nonzero, subregions will be clipped to @size.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @ops: a structure containing read and write callbacks to be used when
* I/O is performed on the region.
* @opaque: passed to the read and write callbacks of the @ops structure.
@@ -1275,7 +1272,7 @@ void memory_region_init_io(MemoryRegion *mr,
* directly.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: size of the region.
@@ -1298,7 +1295,7 @@ bool memory_region_init_ram_nomigrate(MemoryRegion *m=
r,
* modify memory directly.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: size of the region.
@@ -1328,7 +1325,7 @@ bool memory_region_init_ram_flags_nomigrate(MemoryReg=
ion *mr,
* canceled.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: used size of the region.
@@ -1357,7 +1354,7 @@ bool memory_region_init_resizeable_ram(MemoryRegion *=
mr,
* mmap-ed backend.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: size of the region.
@@ -1390,7 +1387,7 @@ bool memory_region_init_ram_from_file(MemoryRegion *m=
r,
* mmap-ed backend.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: the name of the region.
* @size: size of the region.
* @ram_flags: RamBlock flags. Supported flags: RAM_SHARED, RAM_PMEM,
@@ -1421,7 +1418,7 @@ bool memory_region_init_ram_from_fd(MemoryRegion *mr,
* region will modify memory directly.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: size of the region.
@@ -1449,7 +1446,7 @@ void memory_region_init_ram_ptr(MemoryRegion *mr,
* skip_dump flag.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: the name of the region.
* @size: size of the region.
* @ptr: memory to be mapped; must contain at least @size bytes.
@@ -1469,7 +1466,7 @@ void memory_region_init_ram_device_ptr(MemoryRegion *=
mr,
* part of another memory region.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: used for debugging; not visible to the user or ABI
* @orig: the region to be referenced; @mr will be equivalent to
* @orig between @offset and @offset + @size - 1.
@@ -1495,7 +1492,7 @@ void memory_region_init_alias(MemoryRegion *mr,
* of the caller.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: size of the region.
@@ -1518,7 +1515,7 @@ bool memory_region_init_rom_nomigrate(MemoryRegion *m=
r,
* of the caller.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @ops: callbacks for write access handling (must not be NULL).
* @opaque: passed to the read and write callbacks of the @ops structure.
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
@@ -1554,7 +1551,7 @@ bool memory_region_init_rom_device_nomigrate(MemoryRe=
gion *mr,
* @_iommu_mr: the #IOMMUMemoryRegion to be initialized
* @instance_size: the IOMMUMemoryRegion subclass instance size
* @mrtypename: the type name of the #IOMMUMemoryRegion
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: used for debugging; not visible to the user or ABI
* @size: size of the region.
*/
@@ -1570,7 +1567,7 @@ void memory_region_init_iommu(void *_iommu_mr,
* region will modify memory directly.
*
* @mr: the #MemoryRegion to be initialized
- * @owner: the object that tracks the region's reference count (must be
+ * @owner: the object that keeps the region alive (must be
* TYPE_DEVICE or a subclass of TYPE_DEVICE, or NULL)
* @name: name of the memory region
* @size: size of the region in bytes
@@ -1616,7 +1613,7 @@ bool memory_region_init_ram_guest_memfd(MemoryRegion =
*mr,
* If you pass a non-NULL non-device @owner then we will assert.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
* must be unique within any device
* @size: size of the region.
@@ -1647,7 +1644,7 @@ bool memory_region_init_rom(MemoryRegion *mr,
* If you pass a non-NULL non-device @owner then we will assert.
*
* @mr: the #MemoryRegion to be initialized.
- * @owner: the object that tracks the region's reference count
+ * @owner: the object that keeps the region alive
* @ops: callbacks for write access handling (must not be NULL).
* @opaque: passed to the read and write callbacks of the @ops structure.
* @name: Region name, becomes part of RAMBlock name used in migration str=
eam
--=20
2.47.1
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From: Akihiko Odaki <akihiko.odaki@daynix.com>
Date: Thu, 09 Jan 2025 14:50:21 +0900
Subject: [PATCH v7 2/2] memory: Do not create circular reference with
subregion
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References: <20250109-san-v7-0-93c432a73024@daynix.com>
In-Reply-To: <20250109-san-v7-0-93c432a73024@daynix.com>
To: Eduardo Habkost <eduardo@habkost.net>,
Marcel Apfelbaum <marcel.apfelbaum@gmail.com>,
=?utf-8?q?Philippe_Mathieu-Daud=C3=A9?= <philmd@linaro.org>,
Yanan Wang <wangyanan55@huawei.com>, John Snow <jsnow@redhat.com>,
BALATON Zoltan <balaton@eik.bme.hu>, Jiaxun Yang <jiaxun.yang@flygoat.com>,
Nicholas Piggin <npiggin@gmail.com>,
Daniel Henrique Barboza <danielhb413@gmail.com>,
David Gibson <david@gibson.dropbear.id.au>,
Harsh Prateek Bora <harshpb@linux.ibm.com>,
Alexey Kardashevskiy <aik@ozlabs.ru>, "Michael S. Tsirkin" <mst@redhat.com>,
=?utf-8?q?Alex_Benn=C3=A9e?= <alex.bennee@linaro.org>,
Peter Xu <peterx@redhat.com>, Fabiano Rosas <farosas@suse.de>,
Paolo Bonzini <pbonzini@redhat.com>, David Hildenbrand <david@redhat.com>,
Thomas Huth <thuth@redhat.com>, Laurent Vivier <lvivier@redhat.com>,
Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-ppc@nongnu.org,
devel@daynix.com, Akihiko Odaki <akihiko.odaki@daynix.com>
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memory_region_update_container_subregions() used to call
memory_region_ref(), which creates a reference to the owner of the
subregion, on behalf of the owner of the container. This results in a
circular reference if the subregion and container have the same owner.
memory_region_ref() creates a reference to the owner instead of the
memory region to match the lifetime of the owner and memory region. We
do not need such a hack if the subregion and container have the same
owner because the owner will be alive as long as the container is.
Therefore, create a reference to the subregion itself instead ot its
owner in such a case; the reference to the subregion is still necessary
to ensure that the subregion gets finalized after the container.
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
---
system/memory.c | 24 ++++++++++++++++++++++--
1 file changed, 22 insertions(+), 2 deletions(-)
diff --git a/system/memory.c b/system/memory.c
index 78e17e0efa83..972617b0192d 100644
--- a/system/memory.c
+++ b/system/memory.c
@@ -1855,6 +1855,24 @@ void memory_region_unref(MemoryRegion *mr)
}
}
=20
+static void memory_region_ref_subregion(MemoryRegion *subregion)
+{
+ if (subregion->container->owner =3D=3D subregion->owner) {
+ object_ref(OBJECT(subregion));
+ } else {
+ memory_region_ref(subregion);
+ }
+}
+
+static void memory_region_unref_subregion(MemoryRegion *subregion)
+{
+ if (subregion->container->owner =3D=3D subregion->owner) {
+ object_unref(OBJECT(subregion));
+ } else {
+ memory_region_unref(subregion);
+ }
+}
+
uint64_t memory_region_size(MemoryRegion *mr)
{
if (int128_eq(mr->size, int128_2_64())) {
@@ -2644,7 +2662,8 @@ static void memory_region_update_container_subregions=
(MemoryRegion *subregion)
=20
memory_region_transaction_begin();
=20
- memory_region_ref(subregion);
+ memory_region_ref_subregion(subregion);
+
QTAILQ_FOREACH(other, &mr->subregions, subregions_link) {
if (subregion->priority >=3D other->priority) {
QTAILQ_INSERT_BEFORE(other, subregion, subregions_link);
@@ -2702,7 +2721,8 @@ void memory_region_del_subregion(MemoryRegion *mr,
assert(alias->mapped_via_alias >=3D 0);
}
QTAILQ_REMOVE(&mr->subregions, subregion, subregions_link);
- memory_region_unref(subregion);
+ memory_region_unref_subregion(subregion);
+
memory_region_update_pending |=3D mr->enabled && subregion->enabled;
memory_region_transaction_commit();
}
--=20
2.47.1