linux-kernel - Re: [RFC] fsblock

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Date:	Mon, 25 Jun 2007 08:25:21 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Nick Piggin <npiggin@...e.de>, Jeff Garzik <jeff@...zik.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] fsblock

On Mon, Jun 25, 2007 at 04:58:48PM +1000, Nick Piggin wrote:
> 
> >Using buffer heads instead allows the FS to send file data down inside
> >the transaction code, without taking the page lock.  So, locking wrt
> >data=ordered is definitely going to be tricky.
> >
> >The best long term option may be making the locking order
> >transaction -> page lock, and change writepage to punt to some other
> >queue when it needs to start a transaction.
> 
> Yeah, that's what I would like, and I think it would come naturally
> if we move away from these "pass down a single, locked page APIs"
> in the VM, and let the filesystem do the locking and potentially
> batching of larger ranges.

Definitely.

> 
> write_begin/write_end is a step in that direction (and it helps
> OCFS and GFS quite a bit). I think there is also not much reason
> for writepage sites to require the page to lock the page and clear
> the dirty bit themselves (which has seems ugly to me).

If we keep the page mapping information with the page all the time (ie
writepage doesn't have to call get_block ever), it may be possible to
avoid sending down a locked page.  But, I don't know the delayed
allocation internals well enough to say for sure if that is true.

Either way, writepage is the easiest of the bunch because it can be
deferred.

-chris
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