[PATCH v1 1/4] softmmu/physmem: Warn with ram_block_discard_range() on MAP_PRIVATE file mapping

[David Hildenbrand <david@redhat.com>]

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.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 softmmu/physmem.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 6bdd944fe8..27c7219c82 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -3451,6 +3451,24 @@ 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) {
-- 
2.40.1