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

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Message-ID: <387271d8-7ad3-4f56-b6da-b0deb0a6be6b@redhat.com>
Date: Tue, 2 Sep 2025 12:21:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 Chris Li <chrisl@...nel.org>, Barry Song <baohua@...nel.org>,
 Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
 Kemeng Shi <shikemeng@...weicloud.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>, 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 02.09.25 12:18, David Hildenbrand wrote:
> On 22.08.25 21:20, Kairui Song wrote:
>> From: Kairui Song <kasong@...cent.com>
>>
>> Swap cache lookup is lockless, it only increases the reference count
>> of the returned folio. That's not enough to ensure a folio is stable in
>> the swap cache, so the folio could be removed from the swap cache at any
>> time. The caller always has to lock and check the folio before use.
>>
>> Document this as a comment, and introduce a helper for swap cache folio
>> verification with proper sanity checks.
>>
>> Also, sanitize all current users to use this convention, and use the new
>> helper when possible for easier debugging. Some existing callers won't
>> cause any major problem right now, only trivial issues like incorrect
>> readahead statistic (swapin) or wasted loop (swapoff). It's better to
>> always follow this convention to make things robust.
>>
>> Signed-off-by: Kairui Song <kasong@...cent.com>
>> ---
> 
> [...]
> 
>> +/**
>> + * folio_contains_swap - Does this folio contain this swap entry?
>> + * @folio: The folio.
>> + * @entry: The swap entry to check against.
>> + *
>> + * Swap version of folio_contains()
>> + *
>> + * Context: The caller should have the folio locked to ensure
>> + * nothing will move it out of the swap cache.
>> + * Return: true or false.
>> + */
> 
> I appreciate the kerneldoc.
> 
> Intuitively, this should be called "..._swap_entry".
> 
> But I wonder if "contains" is really the right term to use here. It's
> more like that a swap entry "belongs to" (was assigned to) a folio, right?
> 
> Sure, we store the information in the folio, but the "contains" is a bit
> weird.
> 
> folio_matches_swp_entry() maybe?

folio_matches_swap_entry() is what I wanted to say :)

-- 
Cheers

David / dhildenb