linux-kernel - Re: Question regarding concurrent accesses through block device and fs

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Date:	Tue, 3 Mar 2009 14:52:42 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Francis Moreau <francis.moro@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Question regarding concurrent accesses through block device and fs

On Tuesday 03 March 2009 00:30:18 Francis Moreau wrote:
> Nick Piggin <nickpiggin@...oo.com.au> writes:
> > On Monday 02 March 2009 08:07:30 Francis Moreau wrote:
> >> This is the case where I can't find when the metadata are actually
> >> written back to the disk by the flushers. I looked at
> >> writback_inodes() but I fail to find this out.
> >>
> >> Could you point out the place in the code where this happen ?
> >
> > I guess it picks them up via their block device inodes.
>
> Probably but I don't find the actual place.

It was an educated guess ;) I'm quite sure it does.


> I looked at the place where page are normally written back to disk (ie
> in background_writeout()) but I can see only the writeback of data, not
> metadata...

What are you expecting writeback of metadata to look like? To the
core kernel it looks the same as writeback of data.


> >> This sounds very weird to me but I need to learn how things work
> >> before doing any serious comments.
> >
> > Why would they? They just operate on their metadata, and the buffer
> > cache is basically a transparent writeback cache to them.
>
> Well the fact that metadata are written back to disk at an unknown point
> in the time means that we don't know in which order metadata and data
> are written. So it means that data can be written before or after
> metadata or they can be mixed up.

But the cache layer on top of that ensures it *appears* not to be mixed
up. A problem arises when the system crashes in the middle of this, and
we lose that information and see a mixed up filesystem. Hence journalling
filesystems.


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