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| author | David Hildenbrand <david@redhat.com> | 2021-04-06 10:01:25 +0200 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2021-06-15 20:27:37 +0200 |
| commit | cdfa56c551bb48f286cfe1f2daa1083d333ee45d (patch) | |
| tree | c033cf1a1fe6a82ef2a42c4de4dea8a94652cd18 | |
| parent | 7ce18ca0257dac6e3ac92c8bc3d610abbf14bcce (diff) | |
| download | focaccia-qemu-cdfa56c551bb48f286cfe1f2daa1083d333ee45d.tar.gz focaccia-qemu-cdfa56c551bb48f286cfe1f2daa1083d333ee45d.zip | |
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: 06329ccecfa0 ("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>
| -rw-r--r-- | include/qemu/osdep.h | 4 | ||||
| -rw-r--r-- | softmmu/physmem.c | 7 |
2 files changed, 8 insertions, 3 deletions
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h index 236a045671..18a9e3fb4c 100644 --- a/include/qemu/osdep.h +++ b/include/qemu/osdep.h @@ -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 */ diff --git a/softmmu/physmem.c b/softmmu/physmem.c index b78b30e7ba..c0a3c47167 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, 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 " |