[PATCH v3 02/12] softmmu/physmem: Fix ram_block_discard_range() to handle shared anonymous memory

[David Hildenbrand]

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.

Fixes: 06329ccecfa0 ("mem: add share parameter to memory-backend-ram")
Signed-off-by: David Hildenbrand <da...@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