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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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?

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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK