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softmmu/physmem: Warn with ram_block_discard_range() on MAP_PRIVATE file mapping

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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 b17fbbe55c ("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>
This commit is contained in:
David Hildenbrand 2023-07-06 09:56:06 +02:00
parent ac23dd2f29
commit 1d44ff586f
1 changed files with 18 additions and 0 deletions
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18
softmmu/physmem.c
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@ -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) {