* Peter Xu (pet...@redhat.com) wrote:
> On Wed, Jun 28, 2017 at 08:00:20PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilb...@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 <dgilb...@redhat.com>
> > ---
> > exec.c | 28 ++++++++++++++++------------
> > trace-events | 3 +++
> > 2 files changed, 19 insertions(+), 12 deletions(-)
> >
> > diff --git a/exec.c b/exec.c
> > index 69fc5c9b07..4e61226a16 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -3557,6 +3557,7 @@ 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",
> > @@ -3566,23 +3567,26 @@ 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) &&
> > + (rb->fd == -1 || !(rb->flags & RAM_SHARED));
> > + need_fallocate = rb->fd != -1;
> > + if (ret == -1 && need_fallocate) {
>
> (ret will always be -1 when reach here?)
Yes, I was just making the code independent of order.
> > #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> > ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE |
> > FALLOC_FL_KEEP_SIZE,
> > start, length);
> > #endif
> > }
> > + if (need_madvise && (!need_fallocate || (ret == 0))) {
> > +#if defined(CONFIG_MADVISE)
> > + ret = madvise(host_startaddr, length, MADV_DONTNEED);
> > +#endif
> > + }
> > + trace_ram_block_discard_range(rb->idstr, host_startaddr,
> > + need_madvise, need_fallocate, ret);
>
> How about make the check easier by:
>
> if (rb->page_size != qemu_host_page_size ||
> rb->flags & RAM_SHARED) {
> /* Either huge pages or shared memory will contain rb->fd */
> assert(rb->fd);
> fallocate(rb->fd, ...);
> } else {
> madvise();
> }
I've reworked this.
There are situations where you want both (I think!) - for
shared memory, that's not a hugepage, you do an fallocate
to clear the underlying storage, and then do an madvise
to force the local mappings to be cleared.
Dave
> Thanks,
>
> --
> Peter Xu
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
