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

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Date:	Thu, 12 Mar 2009 19:22:53 +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 Thursday 12 March 2009 19:05:39 Francis Moreau wrote:

> > It was an educated guess ;) I'm quite sure it does.
>
> Ok I think I got the idea now. I though block device main purpose was
> to handle block nodes such as /dev/sdx but it isn't.

Well, /dev/sdX access is important, at least to create and fsck the
filesystem ;) But for most Linux users, I think majority of buffercache
access will be by filesystem metadata access.

> >> 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.
>
> I don't know. I was just thinking that since metadata are special since
> they handle critical file system information, the kernel did treat them
> specially.

It is, but you have to look in the filesystems themselves to see that.
There are some exceptions to that -- eg. sync_mapping_buffers in
buffer.c where it writes out dirty metadata buffers that the filesystem
has attached to a file. But that's fsync driven rather than background
writeout. 

> > 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.
>
> Ok I guess I win a new tour in the kernel code ;) to understand how the
> cache layer do that.

Ignore details like crashes, direct IO and coherency between data mappings
and buffercache where things get a bit hairy, and it's just a writeback
cache. The last thing you write to some location will be what you get back
if you read from that location -- regardless of whether it is dirty or clean
or not present when you ask for it (and has to be read from disk).

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