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?)
> #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();
}
Thanks,
--
Peter Xu
