linux-kernel - Re: [PATCH 10/16] fuse: Implement writepages callback

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Message-ID: <51FBD2DF.50506@parallels.com>
Date:	Fri, 2 Aug 2013 19:40:15 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	<riel@...hat.com>, <dev@...allels.com>, <xemul@...allels.com>,
	<fuse-devel@...ts.sourceforge.net>, <bfoster@...hat.com>,
	<linux-kernel@...r.kernel.org>, <jbottomley@...allels.com>,
	<linux-mm@...ck.org>, <viro@...iv.linux.org.uk>,
	<linux-fsdevel@...r.kernel.org>, <akpm@...ux-foundation.org>,
	<fengguang.wu@...el.com>, <devel@...nvz.org>, <mgorman@...e.de>
Subject: Re: [PATCH 10/16] fuse: Implement writepages callback

07/19/2013 08:50 PM, Miklos Szeredi пишет:
> On Sat, Jun 29, 2013 at 09:45:29PM +0400, Maxim Patlasov wrote:
>> From: Pavel Emelyanov <xemul@...nvz.org>
>>
>> The .writepages one is required to make each writeback request carry more than
>> one page on it. The patch enables optimized behaviour unconditionally,
>> i.e. mmap-ed writes will benefit from the patch even if fc->writeback_cache=0.
> I rewrote this a bit, so we won't have to do the thing in two passes, which
> makes it simpler and more robust.  Waiting for page writeback here is wrong
> anyway, see comment above fuse_page_mkwrite().  BTW we had a race there because
> fuse_page_mkwrite() didn't take the page lock.  I've also fixed that up and
> pushed a series containing these patches up to implementing ->writepages() to
>
>    git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git writepages
>
> Passed some trivial testing but more is needed.

Thanks a lot for efforts. The approach you implemented looks promising, 
but it introduces the following assumption: a page cannot become dirty 
before we have a chance to wait on fuse writeback holding the page 
locked. This is already true for mmap-ed writes (due to your fixes) and 
it seems doable for cached writes as well (like we do in 
fuse_perform_write). But the assumption seems to be broken in case of 
direct read from local fs (e.g. ext4) to a memory region mmap-ed to a 
file on fuse fs. See how dio_bio_submit() marks pages dirty by 
bio_set_pages_dirty(). I can't see any solution for this use-case. Do you?

Thanks,
Maxim

>
> I'll get to the rest of the patches next week.
>
> Thanks,
> Miklos
>
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