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

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Message-ID: <m2y6vplzot.fsf@gmail.com>
Date:	Sun, 01 Mar 2009 22:07:30 +0100
From:	Francis Moreau <francis.moro@...il.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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

Nick Piggin <nickpiggin@...oo.com.au> writes:

> On Monday 02 March 2009 01:42:55 Francis Moreau wrote:

[...]

> OK, the "buffercache", the cache of block device contents, is normally
> thought of as metadata when it is being used by the filesystem (eg.
> usually via bread() etc), or data when it is being read/written from
> userspace via /dev/<blockdevice>.
>
> In the former case, the buffer.c/filesystem code together know when a
> metadata buffer is unused (because the filesystem has deallocated it),
> so unmap_underlying_metadata will work there.
>
> And it is insane to have a mounted filesystem and have userspace working
> on the same block device, so unmap_underlying_metadata doesn't have to
> care about that case. (IIRC some filesystem tools can do this, but there
> are obviously a lot of tricks to it)

Thanks for clarifying this.

[...]

> Depends on the filesystem. Many do just use the buffercache as a
> writeback cache for their metadata, and are happy to just let the
> dirty page flushers write it out when it suits them

I guess you're talking about the pdflush threads here.

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 ?

> (or when there are explicit sync instructions given).

yes I see where this happens in these cases.

> Most of the time, these filesystems don't really know or care when
> exactly their metadata is under writeback.

This sounds very weird to me but I need to learn how things work
before doing any serious comments.

thanks
-- 
Francis
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