Re: [Qemu-devel] [PATCH v2 05/16] postcopy: enhance ram_block_discard_range for hugepages

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

* Juan Quintela (quintela@redhat.com) wrote:
> "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> > Unfortunately madvise DONTNEED doesn't work on hugepagetlb
> > so use fallocate(FALLOC_FL_PUNCH_HOLE)
> > qemu_fd_getpagesize only sets the page based off a file
> > if the file is from hugetlbfs.
> >
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> 
> But ...
> 
> 
> > ---
> >  exec.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/exec.c b/exec.c
> > index e040cdf..c25f6b3 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -3324,9 +3324,20 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
> >  
> >          errno = ENOTSUP; /* If we are missing MADVISE etc */
> >  
> > +        if (rb->page_size == qemu_host_page_size) {
> >  #if defined(CONFIG_MADVISE)
> > -        ret = qemu_madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
> > +            ret = qemu_madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
> >  #endif
> > +        } else {
> > +            /* Huge page case  - unfortunately it can't do DONTNEED, but
> > +             * it can do the equivalent by FALLOC_FL_PUNCH_HOLE in the
> > +             * huge page file.
> > +             */
> > +#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> > +            ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> > +                            start, length);
> 
> 
> Why can't we use fallocate() when !CONFIG_MADVISE?
> 
> or even ...
> 
>          if (rb->page_size == qemu_host_page_size) {
> #if defined(CONFIG_MADVISE)
>             ret = qemu_madvise(host_startaddr, length, QEMU_MADV_DONTNEED);
> #endif
>           }
> 
>           if (ret == -1) {
>               /* Huge page case  - unfortunately it can't do DONTNEED, but
>                * it can do the equivalent by FALLOC_FL_PUNCH_HOLE in the
>                * huge page file.
>                */
> #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
>            ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
>                             start, length);
> #endif
>           }

The fallocate only works where we have an fd, e.g. in the hugepage case;
the madvise only works where we have anonymous memory.  So if we don't have
madvise, we can't use fallocate for normal anonymous memory.

Actually, it's much more complicated than that - I've got another patch
that adds support for postcopy with memory that's backed by tmpfs with shared=true
and that also uses fallocate;  I'm trying to decide if it also needs
the madvise.

Dave

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