Re: [Qemu-devel] [PATCH v3 01/29] migrate: Update ram_block_discard_range for shared

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

* Peter Xu (peterx@redhat.com) wrote:
> On Fri, Feb 16, 2018 at 01:15:57PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > The choice of call to discard a block is getting more complicated
> > for other cases.   We use fallocate PUNCH_HOLE in any file cases;
> > it works for both hugepage and for tmpfs.
> > We use the DONTNEED for non-hugepage cases either where they're
> > anonymous or where they're private.
> > 
> > Care should be taken when trying other backing files.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  exec.c       | 60 ++++++++++++++++++++++++++++++++++++++++++++++--------------
> >  trace-events |  3 ++-
> >  2 files changed, 48 insertions(+), 15 deletions(-)
> > 
> > diff --git a/exec.c b/exec.c
> > index e8d7b335b6..b1bb477776 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -3702,6 +3702,7 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
> >      }
> >  
> >      if ((start + length) <= rb->used_length) {
> > +        bool need_madvise, need_fallocate;
> >          uint8_t *host_endaddr = host_startaddr + length;
> >          if ((uintptr_t)host_endaddr & (rb->page_size - 1)) {
> >              error_report("ram_block_discard_range: Unaligned end address: %p",
> > @@ -3711,29 +3712,60 @@ 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)
> > -            /* Note: We need the madvise MADV_DONTNEED behaviour of definitely
> > -             * freeing the page.
> > -             */
> > -            ret = madvise(host_startaddr, length, 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.
> > +        /* The logic here is messy;
> > +         *    madvise DONTNEED fails for hugepages
> > +         *    fallocate works on hugepages and shmem
> > +         */
> > +        need_madvise = (rb->page_size == qemu_host_page_size);
> > +        need_fallocate = rb->fd != -1;
> > +        if (need_fallocate) {
> > +            /* For a file, this causes the area of the file to be zero'd
> > +             * if read, and for hugetlbfs also causes it to be unmapped
> > +             * so a userfault will trigger.
> >               */
> >  #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> >              ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> >                              start, length);
> > +            if (ret) {
> > +                ret = -errno;
> > +                error_report("ram_block_discard_range: Failed to fallocate "
> > +                             "%s:%" PRIx64 " +%zx (%d)",
> > +                             rb->idstr, start, length, ret);
> > +                goto err;
> > +            }
> > +#else
> > +            ret = -ENOSYS;
> > +            error_report("ram_block_discard_range: fallocate not available/file"
> > +                         "%s:%" PRIx64 " +%zx (%d)",
> > +                         rb->idstr, start, length, ret);
> > +            goto err;
> >  #endif
> >          }
> > -        if (ret) {
> > -            ret = -errno;
> > -            error_report("ram_block_discard_range: Failed to discard range "
> > +        if (need_madvise) {
> > +            /* For normal RAM this causes it to be unmapped,
> > +             * for shared memory it causes the local mapping to disappear
> > +             * and to fall back on the file contents (which we just
> > +             * fallocate'd away).
> > +             */
> > +#if defined(CONFIG_MADVISE)
> > +            ret =  madvise(host_startaddr, length, MADV_DONTNEED);
> > +            if (ret) {
> > +                ret = -errno;
> > +                error_report("ram_block_discard_range: Failed to discard range "
> > +                             "%s:%" PRIx64 " +%zx (%d)",
> > +                             rb->idstr, start, length, ret);
> > +                goto err;
> > +            }
> > +#else
> > +            ret = -ENOSYS;
> > +            error_report("ram_block_discard_range: MADVISE not available"
> >                           "%s:%" PRIx64 " +%zx (%d)",
> >                           rb->idstr, start, length, ret);
> > +            goto err;
> > +#endif
> >          }
> > +        trace_ram_block_discard_range(rb->idstr, host_startaddr,
> > +                                      need_madvise, need_fallocate, ret);
> 
> Nit: worth to log the length too if it's named as "range"?

Done.

> Either with/without:
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks.

Dave

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