@@ -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 works 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) {
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. Message-ID: <20230706075612.67404-2-david@redhat.com> Acked-by: Peter Xu <peterx@redhat.com> Tested-by: Mario Casquero <mcasquer@redhat.com> Reviewed-by: Juan Quintela <quintela@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: David Hildenbrand <david@redhat.com> --- softmmu/physmem.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)