linux-kernel - Re: [jffs2] [rfc] fix write deadlock regression

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Message-Id: <1188735203.3834.16.camel@shinybook.infradead.org>
Date:	Sun, 02 Sep 2007 13:13:23 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Jason Lunz <lunz@...ooley.org>,
	lkml <linux-kernel@...r.kernel.org>, jffs-dev@...s.com,
	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [jffs2] [rfc] fix write deadlock regression

Jason, thank you _so_ much for finding the underlying cause of this.

On Sun, 2007-09-02 at 06:20 +0200, Nick Piggin wrote:
> Hmm, thanks for that. It does sound like it is deadlocking via
> commit_write(). OTOH, it seems like it could be using the page
> before it is uptodate -- it _may_ only be dealing with uptodate
> data at that point... but if so, why even read_cache_page at
> all?

jffs2_readpage() is synchronous -- there's no chance that the page won't
be up to date. We're doing this for garbage collection -- if there are
many log entries covering a single page of data, we want to write out a
single replacement which covers the whole page, obsoleting the previous
suboptimal representation of the same data.

> However, it is a regression. So unless David can come up with a
> more satisfactory approach, I guess we'd have to go with your
> patch. 

I think Jason's patch is the best answer for the moment. At some point
in the very near future I want to improve the RAM usage and compression
ratio by dropping the rule that data nodes may not cross page boundaries
-- in which case garbage collection will need to do something other than
reading the page using read_cache_page() and then writing it out again;
it'll probably need to end up using its own internal buffer. But for
now, Jason's patch looks good.

Thanks.

-- 
dwmw2

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