On 11.03.21 17:39, Dr. David Alan Gilbert wrote:
* David Hildenbrand (da...@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 <dgilb...@redhat.com>
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
--
Thanks,
David / dhildenb
