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From: | David Hildenbrand |
Subject: | Re: [PATCH v3 02/12] softmmu/physmem: Fix ram_block_discard_range() to handle shared anonymous memory |
Date: | Thu, 11 Mar 2021 17:45:46 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 |
On 11.03.21 17:39, Dr. David Alan Gilbert wrote:
* David Hildenbrand (david@redhat.com) wrote: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 fails silently and does nothing.OK, I wonder how stable these rules are; is it defined anywhere that it's required?
I had a look at the Linux implementation: it's essentially shmem ... but we don't have an fd exposed, so we cannot use fallocate() ... :)
MADV_REMOVE documents (man):"In the initial implementation, only tmpfs(5) was supported MADV_REMOVE; but since Linux 3.5, any filesystem which supports the fallocate(2) FALLOC_FL_PUNCH_HOLE mode also supports MADV_REMOVE."
Thanks!
Still, Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>Fixes: 06329ccecfa0 ("mem: add share parameter to memory-backend-ram") Signed-off-by: David Hildenbrand <david@redhat.com> --- softmmu/physmem.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/softmmu/physmem.c b/softmmu/physmem.c index 62ea4abbdd..2ba815fec6 100644 --- a/softmmu/physmem.c +++ b/softmmu/physmem.c @@ -3506,6 +3506,7 @@ 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; @@ -3539,7 +3540,11 @@ 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 " -- 2.29.2
-- Thanks, David / dhildenb
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