linux-kernel - Re: [PATCH/RESEND v2 3/5] z3fold: extend compaction function

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Date:   Wed, 11 Jan 2017 11:28:48 -0500
From:   Dan Streetman <ddstreet@...e.org>
To:     Vitaly Wool <vitalywool@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH/RESEND v2 3/5] z3fold: extend compaction function

On Wed, Jan 11, 2017 at 10:06 AM, Vitaly Wool <vitalywool@...il.com> wrote:
> z3fold_compact_page() currently only handles the situation when
> there's a single middle chunk within the z3fold page. However it
> may be worth it to move middle chunk closer to either first or
> last chunk, whichever is there, if the gap between them is big
> enough.
>
> This patch adds the relevant code, using BIG_CHUNK_GAP define as
> a threshold for middle chunk to be worth moving.
>
> Signed-off-by: Vitaly Wool <vitalywool@...il.com>
> ---
>  mm/z3fold.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 98ab01f..fca3310 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -268,6 +268,7 @@ static inline void *mchunk_memmove(struct z3fold_header *zhdr,
>                        zhdr->middle_chunks << CHUNK_SHIFT);
>  }
>
> +#define BIG_CHUNK_GAP  3
>  /* Has to be called with lock held */
>  static int z3fold_compact_page(struct z3fold_header *zhdr)
>  {
> @@ -286,8 +287,31 @@ static int z3fold_compact_page(struct z3fold_header *zhdr)
>                 zhdr->middle_chunks = 0;
>                 zhdr->start_middle = 0;
>                 zhdr->first_num++;
> +               return 1;
>         }
> -       return 1;
> +
> +       /*
> +        * moving data is expensive, so let's only do that if
> +        * there's substantial gain (at least BIG_CHUNK_GAP chunks)
> +        */
> +       if (zhdr->first_chunks != 0 && zhdr->last_chunks == 0 &&
> +           zhdr->start_middle - (zhdr->first_chunks + ZHDR_CHUNKS) >=
> +                       BIG_CHUNK_GAP) {
> +               mchunk_memmove(zhdr, zhdr->first_chunks + 1);
> +               zhdr->start_middle = zhdr->first_chunks + 1;

this should be first_chunks + ZHDR_CHUNKS, not + 1.

> +               return 1;
> +       } else if (zhdr->last_chunks != 0 && zhdr->first_chunks == 0 &&
> +                  TOTAL_CHUNKS - (zhdr->last_chunks + zhdr->start_middle
> +                                       + zhdr->middle_chunks) >=
> +                       BIG_CHUNK_GAP) {
> +               unsigned short new_start = NCHUNKS - zhdr->last_chunks -

this should be TOTAL_CHUNKS, not NCHUNKS.

> +                       zhdr->middle_chunks;
> +               mchunk_memmove(zhdr, new_start);
> +               zhdr->start_middle = new_start;
> +               return 1;
> +       }
> +
> +       return 0;
>  }
>
>  /**
> --
> 2.4.2