linux-ext4 - Re: ftruncate-mmap: pages are lost after writing to mmaped file.

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Message-ID: <20090326081843.GA8207@skywalker>
Date:	Thu, 26 Mar 2009 13:48:43 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"Martin J. Bligh" <mbligh@...igh.org>, linux-ext4@...r.kernel.org,
	Ying Han <yinghan@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, guichaz@...il.com,
	Alex Khesin <alexk@...gle.com>,
	Mike Waychison <mikew@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: ftruncate-mmap: pages are lost after writing to mmaped file.

On Tue, Mar 24, 2009 at 03:07:21PM +0100, Jan Kara wrote:
> On Tue 24-03-09 10:01:45, Chris Mason wrote:
> > On Tue, 2009-03-24 at 14:26 +0100, Jan Kara wrote:
> > > On Tue 24-03-09 13:55:10, Jan Kara wrote:
> > 
> > > >   And one more interesting thing I don't yet fully understand - I see pages
> > > > having PageError() set when they are removed from page cache (and they have
> > > > been faulted in before). It's probably some interaction with pagecache
> > > > readahead...
> > >   Argh... So the problem seems to be that get_block() occasionally returns
> > > ENOSPC and we then discard the dirty data (hmm, we could give at least a
> > > warning for that). I'm not yet sure why getblock behaves like this because
> > > the filesystem seems to have enough space but anyway this seems to be some
> > > strange fs trouble as well.
> > > 
> > 
> > Ouch.  Perhaps the free space is waiting on a journal commit?
>   Yes, exactly. I've already found there's lot of space hold by the
> committing transaction (it can easily hold a few hundred megs or a few gigs
> with larger journal and my UML images aren't that big...). And writepage()
> implementation in ext3 does not have a logic to retry. Also
> block_write_full_page() clears buffers dirty bits so it's not easy to retry
> even if we did it. I'm now looking into how to fix this...

We retry block allocation in ext3_write_begin. And for mmap we should be
doing something similar to ext4_page_mkwrite so that we can be sure
that during writepage we don't need to do block allocation.

-aneesh
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