Re: [Qemu-devel] [RFC 02/29] migrate: Update ram_block_discard_range for shared

[Peter Xu]

On Wed, Jun 28, 2017 at 08:00:20PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> 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 <address@hidden>
> ---
>  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