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softmmu/physmem: Fix ram_block_discard_range() to handle shared anonymous memory 

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: 06329ccecf ("mem: add share parameter to memory-backend-ram")
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210406080126.24010-3-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
David Hildenbrand 2021-04-06 10:01:25 +02:00 committed by Paolo Bonzini
parent 7ce18ca025
commit cdfa56c551
2 changed files with 8 additions and 3 deletions
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4
include/qemu/osdep.h
View File

@ -410,7 +410,7 @@ void qemu_anon_ram_free(void *ptr, size_t size);
#ifdef MADV_REMOVE
#define QEMU_MADV_REMOVE MADV_REMOVE
#else
#define QEMU_MADV_REMOVE QEMU_MADV_INVALID
#define QEMU_MADV_REMOVE QEMU_MADV_DONTNEED
#endif
#elif defined(CONFIG_POSIX_MADVISE)
@ -424,7 +424,7 @@ void qemu_anon_ram_free(void *ptr, size_t size);
#define QEMU_MADV_DONTDUMP QEMU_MADV_INVALID
#define QEMU_MADV_HUGEPAGE QEMU_MADV_INVALID
#define QEMU_MADV_NOHUGEPAGE QEMU_MADV_INVALID
#define QEMU_MADV_REMOVE QEMU_MADV_INVALID
#define QEMU_MADV_REMOVE QEMU_MADV_DONTNEED
#else /* no-op */

7
softmmu/physmem.c
View File

@ -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, QEMU_MADV_REMOVE);
} else {
ret = madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
}
if (ret) {
ret = -errno;
error_report("ram_block_discard_range: Failed to discard range "