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

[David Hildenbrand]

On 11.03.21 17:39, Dr. David Alan Gilbert wrote:
> * 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?

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 <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


--
Thanks,

David / dhildenb