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

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Date: Mon, 18 Dec 2023 06:39:13 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <zhouchengming@...edance.com>, 
	Nhat Pham <nphamcs@...il.com>, Chris Li <chriscli@...gle.com>, 
	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 Mon, Dec 18, 2023 at 6:03 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Thu, Dec 14, 2023 at 02:41:26PM -0800, Yosry Ahmed wrote:
> > 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) {
>
> That's the wrong branch, no?
>
> !page -> -ENOMEM
>
> page && !page_was_allocated -> already in swapcache

Ah yes, my bad.

>
> Personally, I don't really get the comment. What does it mean that
> it's "okay" not to free the entry? There is a put, which may or may
> not free the entry if somebody else is using it. Is it explaining how
> lifetime works for refcounted objects? I'm similarly confused by the
> "it's okay" to return non-zero. What is that trying to convey?
>
> Deletion seemed like the right choice here, IMO ;)

It's not the clearest of comments for sure. I think it is just trying
to say that it is okay not to write back the entry from zswap and to
fail, because the caller will just try another page. I did not like
silently deleting the comment during the refactoring. How about
rewriting it to something like:

/*
 * If we get here because the page is already in the swapcache, a
 * load may be happening concurrently. Skip this page, the caller
 * will move on to a different page.
 */