linux-kernel - Re: [PATCH v5 2/3] z3fold: remove redundant locking

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Date:   Mon, 17 Oct 2016 16:48:58 -0400
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>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v5 2/3] z3fold: remove redundant locking

On Sat, Oct 15, 2016 at 7:59 AM, Vitaly Wool <vitalywool@...il.com> wrote:
> The per-pool z3fold spinlock should generally be taken only when
> a non-atomic pool variable is modified. There's no need to take it
> to map/unmap an object.

no.  it absolutely needs to be taken, because z3fold_compact_page
could move the middle bud's contents to the first bud, and if the
middle bud gets mapped while it's being moved really bad things will
happen.

you can change that to a per-page lock in the z3fold_header, but some
locking needs to happen between mapping and middle bud moving (and
handle encoding/decoding and first_num access).


>
> Signed-off-by: Vitaly Wool <vitalywool@...il.com>
> ---
>  mm/z3fold.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 5197d7b..10513b5 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -580,6 +580,7 @@ next:
>                 if ((test_bit(PAGE_HEADLESS, &page->private) && ret == 0) ||
>                     (zhdr->first_chunks == 0 && zhdr->last_chunks == 0 &&
>                      zhdr->middle_chunks == 0)) {
> +                       spin_unlock(&pool->lock);
>                         /*
>                          * All buddies are now free, free the z3fold page and
>                          * return success.
> @@ -587,7 +588,6 @@ next:
>                         clear_bit(PAGE_HEADLESS, &page->private);
>                         free_z3fold_page(zhdr);
>                         atomic64_dec(&pool->pages_nr);
> -                       spin_unlock(&pool->lock);
>                         return 0;
>                 }  else if (!test_bit(PAGE_HEADLESS, &page->private)) {
>                         if (zhdr->first_chunks != 0 &&
> @@ -629,7 +629,6 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
>         void *addr;
>         enum buddy buddy;
>
> -       spin_lock(&pool->lock);
>         zhdr = handle_to_z3fold_header(handle);
>         addr = zhdr;
>         page = virt_to_page(zhdr);
> @@ -656,7 +655,6 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
>                 break;
>         }
>  out:
> -       spin_unlock(&pool->lock);
>         return addr;
>  }
>
> @@ -671,19 +669,14 @@ static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
>         struct page *page;
>         enum buddy buddy;
>
> -       spin_lock(&pool->lock);
>         zhdr = handle_to_z3fold_header(handle);
>         page = virt_to_page(zhdr);
>
> -       if (test_bit(PAGE_HEADLESS, &page->private)) {
> -               spin_unlock(&pool->lock);
> -               return;
> +       if (!test_bit(PAGE_HEADLESS, &page->private)) {
> +               buddy = handle_to_buddy(handle);
> +               if (buddy == MIDDLE)
> +                       clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
>         }
> -
> -       buddy = handle_to_buddy(handle);
> -       if (buddy == MIDDLE)
> -               clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
> -       spin_unlock(&pool->lock);
>  }
>
>  /**
> --
> 2.4.2