Re: [Qemu-devel] [PATCH v2 05/16] postcopy: enhance ram_block_discard_range for hugepages

[Juan Quintela]

"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
          }