linux-kernel - Re: [PATCH 5/5] swap: avoid ra statistic lost when swapin races

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Date:   Sun, 11 Dec 2022 20:15:02 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 5/5] swap: avoid ra statistic lost when swapin races

Huang, Ying <ying.huang@...el.com> 于2022年12月11日周日 20:03写道：
>
> Kairui Song <ryncsn@...il.com> writes:
>
> > Matthew Wilcox <willy@...radead.org> 于2022年12月9日周五 03:14写道：
> >>
> >
> > Hi, thanks for the review.
> >
> >> On Fri, Dec 09, 2022 at 02:02:09AM +0800, Kairui Song wrote:
> >> > From: Kairui Song <kasong@...cent.com>
> >> >
> >> > __read_swap_cache_async should just call swap_cache_get_folio for trying
> >> > to look up the swap cache. Because swap_cache_get_folio handles the
> >> > readahead statistic, and clears the RA flag, looking up the cache
> >> > directly will skip these parts.
> >> >
> >> > And the comment no longer applies after commit 442701e7058b
> >> > ("mm/swap: remove swap_cache_info statistics"), just remove them.
> >>
> >> But what about the readahead stats?
> >>
> >
> > Shouldn't readahead stats be accounted here? __read_swap_cache_async
> > is called by swap read in path, if it hits the swap cache, and the
> > page have readahead page flag set, then accounting that readahead
> > should be just the right thing todo. And the readahead flag is checked
> > with folio_test_clear_readahead, so there should be no issue about
> > repeated accounting.
> >
> > Only the addr info of the swap_readahead_info could be updated for
> > multiple times by racing readers, but I think that seems fine, since
> > we don't know which swap read comes later in case of race, just let
> > the last reader that hits the swap cache update the address info of
> > readahead makes sense to me.
> >
> > Or do you mean I should update the comment about the readahead stat
> > instead of just drop the commnet?
>
> __read_swap_cache_async() is called by readahead too
> (swap_vma_readahead/__read_swap_cache_async).  I don't think that it's a
> good idea to do swap readahead operation in this function.

Ah, I got it.
Thanks for pointing out the issue, I'll drop this patch.

>
> Best Regards,
> Huang, Ying