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

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Message-ID: <baaef4711002240826t1b447662yf66c980dea1d9d00@mail.gmail.com>
Date:	Wed, 24 Feb 2010 17:26:37 +0100
From:	Camille Moncelier <pix@...life.org>
To:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	ext4 development <linux-ext4@...r.kernel.org>
Cc:	Jan Kara <jack@...e.cz>
Subject: Re: [ext3] Changes to block device after an ext3 mount point has been 
	remounted readonly

> 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.
>
>
I think I don't understand correctly your patch. For me it seems that
when an ext3 filesystem is remounted ro, some data may not have been
written to disk right ?

But as far as I understand some writes are performed on the journal on
remount-ro, before the ro flag is set. So if writepage comes to play
and write data to disk it my have to update the journal again, no ? If
not it would mean that the journal would reference data that aren't
available on disk ?

Last question, would it be hard to implement a patch that trigger
writepage and wait for completion when remounting read-only (I have no
expertise on filesystems in general, but I tried my best to understand
the ext3 driver)

-- 
Camille Moncelier
http://devlife.org/

If Java had true garbage collection, most programs would
delete themselves upon execution.
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