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From: David Hildenbrand <david@redhat.com>
To: qemu-devel@nongnu.org
Cc: David Hildenbrand <david@redhat.com>,
	Igor Kotrasinski <i.kotrasinsk@partner.samsung.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Peter Xu <peterx@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH for-6.0 v1 2/3] softmmu/physmem: Fix ram_block_discard_range() to handle shared anonymous memory
Date: Tue,  6 Apr 2021 10:01:25 +0200	[thread overview]
Message-ID: <20210406080126.24010-3-david@redhat.com> (raw)
In-Reply-To: <20210406080126.24010-1-david@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 <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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 76bb8e324e..afff96a6dc 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 "
-- 
2.30.2



  parent reply	other threads:[~2021-04-06  8:05 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-06  8:01 [PATCH for-6.0 v1 0/3] softmmu/physmem: shared anonymous memory fixes David Hildenbrand
2021-04-06  8:01 ` [PATCH for-6.0 v1 1/3] softmmu/physmem: Mark shared anonymous memory RAM_SHARED David Hildenbrand
2021-04-06  8:01 ` David Hildenbrand [this message]
2021-04-06  8:01 ` [PATCH for-6.0 v1 3/3] softmmu/physmem: Fix qemu_ram_remap() to handle shared anonymous memory David Hildenbrand
2021-04-06 16:46 ` [PATCH for-6.0 v1 0/3] softmmu/physmem: shared anonymous memory fixes Peter Xu
2021-04-07  8:34   ` David Hildenbrand
2021-05-25  8:06 ` David Hildenbrand

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