linux-kernel - Re: [PATCH] f2fs: f2fs supports uncached buffered I/O

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Message-ID: <056e083b-8e41-45a2-9b0f-2ec47d1a9f71@kernel.dk>
Date: Tue, 15 Jul 2025 08:28:37 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Qi Han <hanqi@...o.com>, jaegeuk@...nel.org, chao@...nel.org
Cc: linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] f2fs: f2fs supports uncached buffered I/O

On 7/14/25 9:10 PM, Qi Han wrote:
> Jens has already completed the development of uncached buffered I/O
> in git [1], and in f2fs, the feature can be enabled simply by setting
> the FOP_DONTCACHE flag in f2fs_file_operations.

You need to ensure that for any DONTCACHE IO that the completion is
routed via non-irq context, if applicable. I didn't verify that this is
the case for f2fs. Generally you can deduce this as well through
testing, I'd say the following cases would be interesting to test:

1) Normal DONTCACHE buffered read
2) Overwrite DONTCACHE buffered write
3) Append DONTCACHE buffered write

Test those with DEBUG_ATOMIC_SLEEP set in your config, and it that
doesn't complain, that's a great start.

For the above test cases as well, verify that page cache doesn't grow as
IO is performed. A bit is fine for things like meta data, but generally
you want to see it remain basically flat in terms of page cache usage.

Maybe this is all fine, like I said I didn't verify. Just mentioning it
for completeness sake.

-- 
Jens Axboe