[PATCH V3] memory: RAM_NAMED_FILE flag

[PATCH V3] memory: RAM_NAMED_FILE flag

[Steve Sistare]

migrate_ignore_shared() is an optimization that avoids copying memory
that is visible and can be mapped on the target.  However, a
memory-backend-ram or a memory-backend-memfd block with the RAM_SHARED
flag set is not migrated when migrate_ignore_shared() is true.  This is
wrong, because the block has no named backing store, and its contents will
be lost.  To fix, ignore shared memory iff it is a named file.  Define a
new flag RAM_NAMED_FILE to distinguish this case.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
---
 backends/hostmem-file.c   | 1 +
 include/exec/cpu-common.h | 1 +
 include/exec/memory.h     | 3 +++
 migration/ram.c           | 3 ++-
 qapi/migration.json       | 4 ++--
 softmmu/physmem.c         | 7 ++++++-
 6 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 38ea65b..b4335a8 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -57,6 +57,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
     ram_flags = backend->share ? RAM_SHARED : 0;
     ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
     ram_flags |= fb->is_pmem ? RAM_PMEM : 0;
+    ram_flags |= RAM_NAMED_FILE;
     memory_region_init_ram_from_file(&backend->mr, OBJECT(backend), name,
                                      backend->size, fb->align, ram_flags,
                                      fb->mem_path, fb->offset, fb->readonly,
diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
index e5a55ed..87dc9a7 100644
--- a/include/exec/cpu-common.h
+++ b/include/exec/cpu-common.h
@@ -93,6 +93,7 @@ void qemu_ram_set_uf_zeroable(RAMBlock *rb);
 bool qemu_ram_is_migratable(RAMBlock *rb);
 void qemu_ram_set_migratable(RAMBlock *rb);
 void qemu_ram_unset_migratable(RAMBlock *rb);
+bool qemu_ram_is_named_file(RAMBlock *rb);
 int qemu_ram_get_fd(RAMBlock *rb);
 
 size_t qemu_ram_pagesize(RAMBlock *block);
diff --git a/include/exec/memory.h b/include/exec/memory.h
index c3661b2..47c2e02 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -232,6 +232,9 @@ typedef struct IOMMUTLBEvent {
 /* RAM that isn't accessible through normal means. */
 #define RAM_PROTECTED (1 << 8)
 
+/* RAM is an mmap-ed named file */
+#define RAM_NAMED_FILE (1 << 9)
+
 static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
                                        IOMMUNotifierFlag flags,
                                        hwaddr start, hwaddr end,
diff --git a/migration/ram.c b/migration/ram.c
index 88a6c82..cc98da3 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -197,7 +197,8 @@ static bool postcopy_preempt_active(void)
 bool ramblock_is_ignored(RAMBlock *block)
 {
     return !qemu_ram_is_migratable(block) ||
-           (migrate_ignore_shared() && qemu_ram_is_shared(block));
+           (migrate_ignore_shared() && qemu_ram_is_shared(block) &&
+            qemu_ram_is_named_file(block));
 }
 
 #undef RAMBLOCK_FOREACH
diff --git a/qapi/migration.json b/qapi/migration.json
index 179af0c..5bb5ab8 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -465,8 +465,8 @@
 #     block devices (and thus take locks) immediately at the end of
 #     migration.  (since 3.0)
 #
-# @x-ignore-shared: If enabled, QEMU will not migrate shared memory
-#     (since 4.0)
+# @x-ignore-shared: If enabled, QEMU will not migrate shared memory that is
+#     accessible on the destination machine. (since 4.0)
 #
 # @validate-uuid: Send the UUID of the source to allow the destination
 #     to ensure it is the same.  (since 4.2)
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 9d7e172..a8f742a 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -1570,6 +1570,11 @@ void qemu_ram_unset_migratable(RAMBlock *rb)
     rb->flags &= ~RAM_MIGRATABLE;
 }
 
+bool qemu_ram_is_named_file(RAMBlock *rb)
+{
+    return rb->flags & RAM_NAMED_FILE;
+}
+
 int qemu_ram_get_fd(RAMBlock *rb)
 {
     return rb->fd;
@@ -1880,7 +1885,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
 
     /* Just support these ram flags by now. */
     assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_NORESERVE |
-                          RAM_PROTECTED)) == 0);
+                          RAM_PROTECTED | RAM_NAMED_FILE)) == 0);
 
     if (xen_enabled()) {
         error_setg(errp, "-mem-path not supported with Xen");
-- 
1.8.3.1

Re: [PATCH V3] memory: RAM_NAMED_FILE flag

[Philippe Mathieu-Daudé]

On 7/6/23 17:18, Steve Sistare wrote:
> migrate_ignore_shared() is an optimization that avoids copying memory
> that is visible and can be mapped on the target.  However, a
> memory-backend-ram or a memory-backend-memfd block with the RAM_SHARED
> flag set is not migrated when migrate_ignore_shared() is true.  This is
> wrong, because the block has no named backing store, and its contents will
> be lost.  To fix, ignore shared memory iff it is a named file.  Define a
> new flag RAM_NAMED_FILE to distinguish this case.
> 
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> ---
>   backends/hostmem-file.c   | 1 +
>   include/exec/cpu-common.h | 1 +
>   include/exec/memory.h     | 3 +++
>   migration/ram.c           | 3 ++-
>   qapi/migration.json       | 4 ++--
>   softmmu/physmem.c         | 7 ++++++-
>   6 files changed, 15 insertions(+), 4 deletions(-)

I'm taking this in my next PR, thanks.