linux-kernel - Re: [PATCH 5/6] iomap: drop unnecessary state

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Message-ID: <995196b3-3571-b23f-eb5f-d3fee5d97593@huaweicloud.com>
Date: Thu, 1 Aug 2024 09:52:49 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, djwong@...nel.org, hch@...radead.org,
 brauner@...nel.org, david@...morbit.com, jack@...e.cz, yi.zhang@...wei.com,
 chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH 5/6] iomap: drop unnecessary state_lock when setting ifs
 uptodate bits

On 2024/8/1 0:52, Matthew Wilcox wrote:
> On Wed, Jul 31, 2024 at 05:13:04PM +0800, Zhang Yi wrote:
>> Commit '1cea335d1db1 ("iomap: fix sub-page uptodate handling")' fix a
>> race issue when submitting multiple read bios for a page spans more than
>> one file system block by adding a spinlock(which names state_lock now)
>> to make the page uptodate synchronous. However, the race condition only
>> happened between the read I/O submitting and completeing threads, it's
>> sufficient to use page lock to protect other paths, e.g. buffered write
>> path. After large folio is supported, the spinlock could affect more
>> about the buffered write performance, so drop it could reduce some
>> unnecessary locking overhead.
> 
> This patch doesn't work.  If we get two read completions at the same
> time for blocks belonging to the same folio, they will both write to
> the uptodate array at the same time.
> 
This patch just drop the state_lock in the buffered write path, doesn't
affect the read path, the uptodate setting in the read completion path
is still protected the state_lock, please see iomap_finish_folio_read().
So I think this patch doesn't affect the case you mentioned, or am I
missing something?

Thanks,
Yi.