From cdfa56c551bb48f286cfe1f2daa1083d333ee45d Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Tue, 6 Apr 2021 10:01:25 +0200 Subject: [PATCH] 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 Reviewed-by: Dr. David Alan Gilbert Signed-off-by: David Hildenbrand Message-Id: <20210406080126.24010-3-david@redhat.com> Signed-off-by: Paolo Bonzini --- include/qemu/osdep.h | 4 ++-- 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 "