linux-ext4 - Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly

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Message-ID: <877hq2tyg8.fsf@openvz.org>
Date:	Wed, 24 Feb 2010 19:01:27 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Camille Moncelier <pix@...life.org>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly

Jan Kara <jack@...e.cz> writes:

>> The fact is that I've been able to reproduce the problem on LVM block
>> devices, and sd* block devices so it's definitely not a loop device
>> specific problem.
>> 
>> By the way, I tried several other things other than "echo s
>> >/proc/sysrq_trigger" I tried multiple sync followed with a one minute
>> "sleep",
>> 
>> "echo 3 >/proc/sys/vm/drop_caches" seems to lower the chances of "hash
>> changes" but doesn't stops them.
>   Strange. When I use sync(1) in your script and use /dev/sda5 instead of a
> /dev/loop0, I cannot reproduce the problem (was running the script for
> something like an hour).
Theoretically some pages may exist after rw=>ro remount
because of generic race between write/sync, And they will be written
in by writepage if page already has buffers. This not happen in ext4
because. Each time it try to perform writepages it try to start_journal
and this result in EROFS.
The race bug will be closed some day but new one may appear again.

Let's be honest and change ext3 writepage like follows:
- check ROFS flag inside write page
- dump writepage's errors.


View attachment "0001-ext3-add-sanity-checks-to-writeback.patch" of type "text/plain" (3725 bytes)