@@ -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 "
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(-)