From: David Hildenbrand <da...@redhat.com>
We can create shared anonymous memory via
"-object memory-backend-ram,share=on,..."
which is, for example, required by PVRDMA for mremap() to work.
Shared anonymous memory is weird, though. Instead of MADV_DONTNEED, we
have to use MADV_REMOVE: MADV_DONTNEED will only remove / zap all
relevant page table entries of the current process, the backend storage
will not get removed, resulting in no reduced memory consumption and
a repopulation of previous content on next access.
Shared anonymous memory is internally really just shmem, but without a
fd exposed. As we cannot use fallocate() without the fd to discard the
backing storage, MADV_REMOVE gets the same job done without a fd as
documented in "man 2 madvise". Removing backing storage implicitly
invalidates all page table entries with relevant mappings - an additional
MADV_DONTNEED is not required.
Fixes: 06329ccecfa0 ("mem: add share parameter to memory-backend-ram")
Reviewed-by: Peter Xu <pet...@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
Signed-off-by: David Hildenbrand <da...@redhat.com>
Message-Id: <20210406080126.24010-3-da...@redhat.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
softmmu/physmem.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index b78b30e7ba..b417f9c282 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -3527,6 +3527,7 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start,
size_t length)
/* The logic here is messy;
* madvise DONTNEED fails for hugepages
* fallocate works on hugepages and shmem
+ * shared anonymous memory requires madvise REMOVE
*/
need_madvise = (rb->page_size == qemu_host_page_size);
need_fallocate = rb->fd != -1;
@@ -3560,7 +3561,11 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t
start, size_t length)
* fallocate'd away).
*/
#if defined(CONFIG_MADVISE)
- ret = madvise(host_startaddr, length, MADV_DONTNEED);
+ if (qemu_ram_is_shared(rb) && rb->fd < 0) {
+ ret = madvise(host_startaddr, length, MADV_REMOVE);
+ } else {
+ ret = madvise(host_startaddr, length, MADV_DONTNEED);
+ }
if (ret) {
ret = -errno;
error_report("ram_block_discard_range: Failed to discard range
"
--
2.31.1
