linux-kernel - Re: [PATCH 0/4] mm/filemap: Limit page cache size to that supported by xarray

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Date: Thu, 27 Jun 2024 09:48:30 +1000
From: Gavin Shan <gshan@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 djwong@...nel.org, hughd@...gle.com, torvalds@...ux-foundation.org,
 zhenyzha@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH 0/4] mm/filemap: Limit page cache size to that supported
 by xarray

On 6/27/24 6:54 AM, Matthew Wilcox wrote:
> On Wed, Jun 26, 2024 at 10:37:00AM +1000, Gavin Shan wrote:
>> On 6/26/24 5:05 AM, David Hildenbrand wrote:
>>> On 25.06.24 20:58, Andrew Morton wrote:
>>>> On Tue, 25 Jun 2024 20:51:13 +0200 David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>>>> I could split them and feed 1&2 into 6.10-rcX and 3&4 into 6.11-rc1.  A
>>>>>> problem with this approach is that we're putting a basically untested
>>>>>> combination into -stable: 1&2 might have bugs which were accidentally
>>>>>> fixed in 3&4.  A way to avoid this is to add cc:stable to all four
>>>>>> patches.
>>>>>>
>>>>>> What are your thoughts on this matter?
>>>>>
>>>>> Especially 4 should also be CC stable, so likely we should just do it
>>>>> for all of them.
>>>>
>>>> Fine.  A Fixes: for 3 & 4 would be good.  Otherwise we're potentially
>>>> asking for those to be backported further than 1 & 2, which seems
>>>> wrong.
>>>
>>> 4 is shmem fix, which likely dates back a bit longer.
>>>
>>>>
>>>> Then again, by having different Fixes: in the various patches we're
>>>> suggesting that people split the patch series apart as they slot things
>>>> into the indicated places.  In other words, it's not a patch series at
>>>> all - it's a sprinkle of independent fixes.  Are we OK thinking of it
>>>> in that fashion?
>>>
>>> The common themes is "pagecache cannot handle > order-11", #1-3 tackle "ordinary" file THP, #4 tackles shmem THP.
>>>
>>> So I'm not sure we should be splitting it apart. It's just that shmem THP arrived before file THP :)
>>>
>>
>> I rechecked the history, it's a bit hard to have precise fix tag for PATCH[4].
>> Please let me know if you have a better one for PATCH[4].
>>
>> #4
>>    Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
>>    Cc: stable@...nel.org # v4.10+
>>    Fixes: 552446a41661 ("shmem: Convert shmem_add_to_page_cache to XArray")
>>    Cc: stable@...nel.org # v4.20+
>> #3
>>    Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
>>    Cc: stable@...nel.org # v5.18+
>> #2
>>    Fixes: 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
>>    Cc: stable@...nel.org # v5.18+
>> #1
>>    Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
>>    Cc: stable@...nel.org # v5.18+
> 
> I actually think it's this:
> 
> commit 6b24ca4a1a8d
> Author: Matthew Wilcox (Oracle) <willy@...radead.org>
> Date:   Sat Jun 27 22:19:08 2020 -0400
> 
>      mm: Use multi-index entries in the page cache
> 
>      We currently store large folios as 2^N consecutive entries.  While this
>      consumes rather more memory than necessary, it also turns out to be buggy.
>      A writeback operation which starts within a tail page of a dirty folio will
>      not write back the folio as the xarray's dirty bit is only set on the
>      head index.  With multi-index entries, the dirty bit will be found no
>      matter where in the folio the operation starts.
> 
>      This does end up simplifying the page cache slightly, although not as
>      much as I had hoped.
> 
>      Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>      Reviewed-by: William Kucharski <william.kucharski@...cle.com>
> 
> Before this, we could split an arbitrary size folio to order 0.  After
> it, we're limited to whatever the xarray allows us to split.
> 

Thanks, PATCH[4]'s fix tag will point to 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache"),
which was merged to v5.17. The fix tags for other patches are correct

Thanks,
Gavin