linux-kernel - Re: [PATCH 22/43] CacheFiles: Add a hook to write a single page of data to an inode [ver #46]

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Date:	Fri, 3 Apr 2009 04:07:54 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk,
	nfsv4@...ux-nfs.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 22/43] CacheFiles: Add a hook to write a single page of data to an inode [ver #46]

On Friday 03 April 2009 03:55:05 Christoph Hellwig wrote:
> On Fri, Apr 03, 2009 at 03:47:20AM +1100, Nick Piggin wrote:
> > Well they now are quite well filesystem defined. We no longer take
> > the page lock before calling them. Not saying it's perfect, but if
> > the backing fs is just using a known subset of ones that work
> > (like loop does).
> 
> The page lock doesn't matter.  What matters is locks protecting the
> io.  Like the XFS iolock or cluster locks in the cluster filesystems,
> and you will get silent data corruption that way.

Hmm, I can see i_mutex being a problem, but can't see how a filesystem
takes any other locks down that chain?

Naturally a random in-kernel user misses other important things, so yes
a simple write sounds like the best option.


> > Probably yes. But it seems like it should have more discussion IMO
> > (unless it has already been had and I missed it).
> 
> This came up plenty of times.

I mean, unless the discussion agreed on write_one_page being the right
API to add, then it should not be added in fscache and fscache should
just take a workaround for the meantime.


> > I don't think "write_one_page" sounds like a particularly good new
> > API addition.
> 
> I also thing it's not a nice one.  I still haven't seen a really good
> explanation of why it can't just use plain ->write

Good question.
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