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

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Date:   Sun, 11 Dec 2022 20:02:45 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Kairui Song <ryncsn@...il.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

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.

Best Regards,
Huang, Ying