ram_block_discard_range() cannot possibly do the right thing in
MAP_PRIVATE file mappings in the general case.
To achieve the documented semantics, we also have to punch a hole into
the file, possibly messing with other MAP_PRIVATE/MAP_SHARED mappings
of such a file.
For example, using VM templating -- see commit b17fbbe55cba ("migration:
allow private destination ram with x-ignore-shared") -- in combination with
any mechanism that relies on discarding of RAM is problematic. This
includes:
* Postcopy live migration
* virtio-balloon inflation/deflation or free-page-reporting
* virtio-mem
So at least warn that there is something possibly dangerous is going on
when using ram_block_discard_range() in these cases.
Acked-by: Peter Xu <pet...@redhat.com>
Tested-by: Mario Casquero <mcasq...@redhat.com>
Signed-off-by: David Hildenbrand <da...@redhat.com>
---
softmmu/physmem.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index bda475a719..4ee157bda4 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -3456,6 +3456,24 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t
start, size_t length)
* so a userfault will trigger.
*/
#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
+ /*
+ * We'll discard data from the actual file, even though we only
+ * have a MAP_PRIVATE mapping, possibly messing with other
+ * MAP_PRIVATE/MAP_SHARED mappings. There is no easy way to
+ * change that behavior whithout violating the promised
+ * semantics of ram_block_discard_range().
+ *
+ * Only warn, because it work as long as nobody else uses that
+ * file.
+ */
+ if (!qemu_ram_is_shared(rb)) {
+ warn_report_once("ram_block_discard_range: Discarding RAM"
+ " in private file mappings is possibly"
+ " dangerous, because it will modify the"
+ " underlying file and will affect other"
+ " users of the file");
+ }
+
ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
start, length);
if (ret) {
--
2.41.0
