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Re: [Qemu-devel] [PATCH v2 05/16] postcopy: enhance ram_block_discard_ra
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v2 05/16] postcopy: enhance ram_block_discard_range for hugepages |
Date: |
Fri, 24 Feb 2017 13:44:40 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
* Juan Quintela (address@hidden) wrote:
> "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > 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 <address@hidden>
>
> Reviewed-by: Juan Quintela <address@hidden>
>
> 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 / address@hidden / Manchester, UK
- [Qemu-devel] [PATCH v2 00/16] Postcopy: Hugepage support, Dr. David Alan Gilbert (git), 2017/02/06
- [Qemu-devel] [PATCH v2 03/16] postcopy: Chunk discards for hugepages, Dr. David Alan Gilbert (git), 2017/02/06
- [Qemu-devel] [PATCH v2 01/16] postcopy: Transmit ram size summary word, Dr. David Alan Gilbert (git), 2017/02/06
- [Qemu-devel] [PATCH v2 05/16] postcopy: enhance ram_block_discard_range for hugepages, Dr. David Alan Gilbert (git), 2017/02/06
- [Qemu-devel] [PATCH v2 06/16] Fold postcopy_ram_discard_range into ram_discard_range, Dr. David Alan Gilbert (git), 2017/02/06
- [Qemu-devel] [PATCH v2 04/16] exec: ram_block_discard_range, Dr. David Alan Gilbert (git), 2017/02/06