linux-kernel - Re: [RFC] block integrity: Fix write after checksum calculation problem

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Date:	Mon, 11 Apr 2011 20:57:19 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Jeff Layton <jlayton@...hat.com>, djwong <djwong@...ibm.com>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Joel Becker <jlbec@...lplan.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Mingming Cao <mcao@...ibm.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [RFC] block integrity: Fix write after checksum calculation
 problem

On Mon, Apr 11, 2011 at 05:46:52PM -0700, Mingming Cao wrote:
> Oh, right. Currently ext4_page_mkwrite drops the page lock before
> calling it's dirty the page (by write_begin() and write_end().  I
> suspect regrab the lock() after write_end() (with your proposed change)
> and returning with locked still leave the dirty by ext4_page_mkwrite
> unlocked.  We probably should to keep the page locked the page during
> the entire ext4_page_mkwrite() call. Any reason to drop the page lock()
> before calling aops->write_begin()?

write_begin takes the page lock by itself.  That's one of the reasons why
block_page_mkwrite doesn't use plain ->write_begin / write_end, the
other beeing that we already get a page passed to use, so there's no
need to do the pagecache lookup or allocation done by
grab_cache_page_write_begin.

The best thing would be to completely drop ext4's current version
of page_mkwrite and start out with a copy of block_page_mkwrite which
has the journalling calls added back into it.
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