linux-kernel - Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio before use

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Message-ID: <c12b9236-903f-4a8f-aa9e-fd4d9ca1bffd@linux.alibaba.com>
Date: Thu, 28 Aug 2025 11:41:37 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Kairui Song <ryncsn@...il.com>, Chris Li <chrisl@...nel.org>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 Barry Song <baohua@...nel.org>, Baoquan He <bhe@...hat.com>,
 Nhat Pham <nphamcs@...il.com>, Kemeng Shi <shikemeng@...weicloud.com>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...hat.com>,
 Yosry Ahmed <yosryahmed@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio
 before use



On 2025/8/27 22:35, Kairui Song wrote:
> On Wed, Aug 27, 2025 at 4:21 PM Chris Li <chrisl@...nel.org> wrote:
>>
>> On Fri, Aug 22, 2025 at 12:21 PM Kairui Song <ryncsn@...il.com> wrote:
>>
>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>> index e9d0d2784cd5..b4d39f2a1e0a 100644
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@ -2379,8 +2379,6 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>>>                          count_vm_event(PGMAJFAULT);
>>>                          count_memcg_event_mm(fault_mm, PGMAJFAULT);
>>>                  }
>>> -       } else {
>>> -               swap_update_readahead(folio, NULL, 0);
>>
>> Also this update readahead move to later might have a similar problem.
>> All the bail out in the move will lose the readahead status update.
>>
>> The readahead deed is already done. Missing the status update seems
>> incorrect.
> 
> Thanks for the detailed review.
> 
> The only change I wanted here is that swap readahead update should be
> done after checking the folio still corresponds to the swap entry
> triggering the swapin. That should have slight to none effect compared
> to before considering the extremely tiny time window. We are only
> following the convention more strictly.
> 
> In theory it might even help to reduce false updates: if the folio no
> longer corresponds to the swap entry, we are hitting an unrelated
> folio, doing a readahead update will either mislead vma readahead's
> address hint, or could clean up the readahead flag of an unrelated
> folio without actually using it. If the folio does get hit in the
> future, due to the missing readahead flag, the statistic will go
> wrong.

Yes, that’s what I thought as well.

By the way, can we do it right all at once in patch 1 (I mean the shmem 
changes)?