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

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Message-Id: <20240625113720.a2fa982b5cb220b1068e5177@linux-foundation.org>
Date: Tue, 25 Jun 2024 11:37:20 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Gavin Shan <gshan@...hat.com>
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, david@...hat.com, djwong@...nel.org,
 willy@...radead.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 Tue, 25 Jun 2024 19:06:42 +1000 Gavin Shan <gshan@...hat.com> wrote:

> Currently, xarray can't support arbitrary page cache size. More details
> can be found from the WARN_ON() statement in xas_split_alloc(). In our
> test whose code is attached below, we hit the WARN_ON() on ARM64 system
> where the base page size is 64KB and huge page size is 512MB. The issue
> was reported long time ago and some discussions on it can be found here
> [1].
> 
> [1] https://www.spinics.net/lists/linux-xfs/msg75404.html 
> 
> In order to fix the issue, we need to adjust MAX_PAGECACHE_ORDER to one
> supported by xarray and avoid PMD-sized page cache if needed. The code
> changes are suggested by David Hildenbrand.
> 
> PATCH[1] adjusts MAX_PAGECACHE_ORDER to that supported by xarray
> PATCH[2-3] avoids PMD-sized page cache in the synchronous readahead path
> PATCH[4] avoids PMD-sized page cache for shmem files if needed

Questions on the timing of these.

1&2 are cc:stable whereas 3&4 are not.

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?