* Juan Quintela (quint...@redhat.com) wrote:
> "Dr. David Alan Gilbert (git)" <dgilb...@redhat.com> wrote:
> > From: "Dr. David Alan Gilbert" <dgilb...@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 <dgilb...@redhat.com>
>
> Reviewed-by: Juan Quintela <quint...@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 / dgilb...@redhat.com / Manchester, UK
