linux-kernel - Re: [PATCH v2 4/6] filemap: do not use folio

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Message-ID: <a462d132-3673-435e-9deb-f81278a65b8d@redhat.com>
Date: Tue, 29 Apr 2025 15:43:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 Chris Li <chrisl@...nel.org>, Yosry Ahmed <yosryahmed@...gle.com>,
 "Huang, Ying" <ying.huang@...ux.alibaba.com>, Nhat Pham <nphamcs@...il.com>,
 Johannes Weiner <hannes@...xchg.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] filemap: do not use folio_contains for swap cache
 folios

On 29.04.25 15:32, Kairui Song wrote:
> On Tue, Apr 29, 2025 at 8:22 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 29.04.25 13:49, Kairui Song wrote:
>>> From: Kairui Song <kasong@...cent.com>
>>>
>>> Currently, none of the folio_contains callers should encounter swap
>>> cache folios.
>>>
>>> For fs/ callers, swap cache folios are never part of their workflow.
>>>
>>> For filemap and truncate, folio_contains is only used for sanity
>>> checks to verify the folio index matches the expected
>>> lookup / invalidation target.
>>>
>>> The swap cache does not utilize filemap or truncate helpers in ways
>>> that would trigger these checks, as it mostly implements its own
>>> cache management.
>>>
>>> Shmem won't trigger these sanity checks either unless thing went
>>> wrong, as it would directly trigger a BUG because swap cache index are
>>> unrelated and almost never matches shmem index. Shmem have to handle
>>> mixed values of folios, shadows, and swap entries, so it has its own
>>> way of handling the mapping.
>>>
>>> While some filemap helpers works for swap cache space, the swap cache
>>> is different from the page cache in many ways. So this particular helper
>>> will unlikely to work in a helpful way for swap cache folios.
>>>
>>> So make it explicit here that folio_contains should not be used for
>>> swap cache folios. This helps to avoid misuse, make swap cache less
>>> exposed and remove the folio_index usage here.
>>>
>>> Signed-off-by: Kairui Song <kasong@...cent.com>
>>> ---
>>>    include/linux/pagemap.h | 8 ++++----
>>>    1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>>> index af25fb640463..1dc3416a9c0d 100644
>>> --- a/include/linux/pagemap.h
>>> +++ b/include/linux/pagemap.h
>>> @@ -935,14 +935,14 @@ static inline struct page *folio_file_page(struct folio *folio, pgoff_t index)
>>>     * @folio: The folio.
>>>     * @index: The page index within the file.
>>>     *
>>> - * Context: The caller should have the page locked in order to prevent
>>> - * (eg) shmem from moving the page between the page cache and swap cache
>>> - * and changing its index in the middle of the operation.
>>> + * Context: The caller should have the folio locked and ensure
>>> + * (e.g.) shmem did not move this folio to swap cache.
>>
>> The "(e.g.)" looks weird. Maybe "ensure that e.g., shmem ..."
>>
>> "to the"
>>
>>>     * Return: true or false.
>>>     */
>>>    static inline bool folio_contains(struct folio *folio, pgoff_t index)
>>>    {
>>> -     return index - folio_index(folio) < folio_nr_pages(folio);
>>> +     VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
>>
>> Likely you want VM_WARN_ON_ONCE_FOLIO() here.
> 
> All its caller will trigger a bug if it encounters a swap cache, so I
> kept that behaviour consistent. Let's keep that unchanged for now.

I suggest reading coding-style.rst about "Do not add new code that uses 
any of the BUG() variants, such as BUG(), BUG_ON(), or VM_BUG_ON()".

VM_BUG_ON is particularly stupid.

-- 
Cheers,

David / dhildenb