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

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Message-ID: <20090324152959.GG23439@duck.suse.cz>
Date:	Tue, 24 Mar 2009 16:29:59 +0100
From:	Jan Kara <jack@...e.cz>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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>
Subject: Re: ftruncate-mmap: pages are lost after writing to mmaped file.

On Tue 24-03-09 15:56:03, Peter Zijlstra wrote:
> On Tue, 2009-03-24 at 15:47 +0100, Jan Kara wrote:
> > 
> > Or we could implement ext3_mkwrite() to allocate buffers already when we
> > make page writeable. But it costs some performace (we have to write page
> > full of zeros when allocating those buffers, where previously we didn't
> > have to do anything) and it's not trivial to make it work if pagesize >
> > blocksize (we should not allocate buffers outside of i_size so if i_size
> > = 1024, we create just one block in ext3_mkwrite() but then we need to
> > allocate more when we extend the file).
> 
> I think this is the best option, failing with SIGBUS when we fail to
> allocate blocks seems consistent with other filesystems as well.
  I agree this looks attractive at the first sight. But there are drawbacks
as I wrote - the problem with blocksize < pagesize, slight performance
decrease due to additional write, page faults doing allocation can take a
*long* time and overall fragmentation is going to be higher (previously
writepage wrote pages for us in the right order, now we are going to
allocate in the first-accessed order). So I'm not sure we really want to
go this way.
  Hmm, maybe we could play a trick ala delayed allocation - i.e., reserve
some space in mkwrite() but don't actually allocate it. That would be done
in writepage(). This would solve all the problems I describe above. We could
use PG_Checked flag to track that the page has a reservation and behave
accordingly in writepage() / invalidatepage(). ext3 in data=journal mode
already uses the flag but the use seems to be compatible with what I want
to do now... So it may actually work.
  BTW: Note that there's a plenty of filesystems that don't implement
mkwrite() (e.g. ext2, UDF, VFAT...) and thus have the same problem with
ENOSPC. So I'd not speak too much about consistency ;).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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