linux-kernel - Re: [PATCH 5/5] mm/zswap: cleanup zswap_reclaim

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Date: Thu, 14 Dec 2023 14:41:26 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Chengming Zhou <zhouchengming@...edance.com>, Nhat Pham <nphamcs@...il.com>, 
	Chris Li <chriscli@...gle.com>, Johannes Weiner <hannes@...xchg.org>, 
	Seth Jennings <sjenning@...hat.com>, Dan Streetman <ddstreet@...e.org>, 
	Vitaly Wool <vitaly.wool@...sulko.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 5/5] mm/zswap: cleanup zswap_reclaim_entry()

On Thu, Dec 14, 2023 at 2:23 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 13 Dec 2023 17:02:25 -0800 Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> > On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> > <zhouchengming@...edance.com> wrote:
> > >
> > > Also after the common decompress part goes to __zswap_load(), we can
> > > cleanup the zswap_reclaim_entry() a little.
> >
> > I think you mean zswap_writeback_entry(), same for the commit title.
>
> I updated my copy of the changelog, thanks.
>
> > > -       /*
> > > -        * If we get here because the page is already in swapcache, a
> > > -        * load may be happening concurrently. It is safe and okay to
> > > -        * not free the entry. It is also okay to return !0.
> > > -        */
> >
> > This comment should be moved above the failure check of
> > __read_swap_cache_async() above, not completely removed.
>
> This?

Yes, thanks a lot. Although I think a new version is needed anyway to
address other comments.

>
> --- a/mm/zswap.c~mm-zswap-cleanup-zswap_reclaim_entry-fix
> +++ a/mm/zswap.c
> @@ -1457,8 +1457,14 @@ static int zswap_writeback_entry(struct
>         mpol = get_task_policy(current);
>         page = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
>                                 NO_INTERLEAVE_INDEX, &page_was_allocated, true);
> -       if (!page)
> +       if (!page) {
> +               /*
> +                * If we get here because the page is already in swapcache, a
> +                * load may be happening concurrently. It is safe and okay to
> +                * not free the entry. It is also okay to return !0.
> +                */
>                 return -ENOMEM;
> +       }
>
>         /* Found an existing page, we raced with load/swapin */
>         if (!page_was_allocated) {
>