linux-ext4 - Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending DIO write race with buffered read

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Date: Tue, 12 Dec 2023 15:25:43 +0100
From: Jan Kara <jack@...e.cz>
To: Matthew Wilcox <willy@...radead.org>
Cc: Jan Kara <jack@...e.cz>, Baokun Li <libaokun1@...wei.com>,
	linux-mm@...ck.org, linux-ext4@...r.kernel.org, tytso@....edu,
	adilger.kernel@...ger.ca, akpm@...ux-foundation.org,
	ritesh.list@...il.com, linux-kernel@...r.kernel.org,
	yi.zhang@...wei.com, yangerkun@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending
 DIO write race with buffered read

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@...e.com>
SUSE Labs, CR