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Date: Tue, 12 Dec 2023 15:25:43 +0100
From: Jan Kara <jack@suse.cz>
To: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>, Baokun Li <libaokun1@huawei.com>,
	linux-mm@kvack.org, linux-ext4@vger.kernel.org, tytso@mit.edu,
	adilger.kernel@dilger.ca, akpm@linux-foundation.org,
	ritesh.list@gmail.com, linux-kernel@vger.kernel.org,
	yi.zhang@huawei.com, yangerkun@huawei.com, yukuai3@huawei.com
Subject: Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending
 DIO write race with buffered read
Message-ID: <20231212142543.jrortgje22traesa@quack3>
References: <20231202091432.8349-1-libaokun1@huawei.com>
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On Tue 12-12-23 04:36:45, Matthew Wilcox wrote:
> On Wed, Dec 06, 2023 at 08:37:57PM +0100, Jan Kara wrote:
> > Within the same page buffered reads and writes should be consistent because
> > they are synchronized by the page lock. However once reads and writes
> > involve multiple pages, there is no serialization so you can get contents
> > of some pages before write and some pages after being written. However this
> > doesn't seem to be your particular case here. I just wanted to point out
> > that in general even buffered reads vs writes are not fully consistent.
> 
> Buffered reads don't take the page/folio lock.  We only use the folio
> lock to avoid reading stale data from the page cache while we're
> fetching the data from storage.  Once the uptodate flag is set on the
> folio, we never take the folio lock for reads.

Right, I've noticed once I've reread the code but thanks for correction
anyway. :)

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR