linux-kernel - Re: [PATCH 2/2] fuse: wait for writeback in fuse_file

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Date:	Fri, 30 Aug 2013 15:33:39 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Brian Foster <bfoster@...hat.com>,
	Pavel Emelianov <xemul@...allels.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<devel@...nvz.org>
Subject: Re: [PATCH 2/2] fuse: wait for writeback in fuse_file_fallocate()
 -v2

08/30/2013 01:13 PM, Miklos Szeredi пишет:
> On Thu, Aug 29, 2013 at 6:41 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>> BTW, isn't it enough to do the filemap_write_and_wait() *plus* the
>> fuse_set_nowrite()?
> Thought about it a bit and I think this should do fine.
>
> Any writes before the fallocate will go trough before the fallocate.
> i_mutex guarantees that only one instance of fuse_set_nowrite() is
> running.  Any mmaped writes during the fallocate() will go after the
> fallocate request and the page cache truncation and that's fine too.
> Page cache is consistent since it doens't contain pages for those
> writes to the hole.  Subsequent reads to that area will fill them in.
>
> Any other concerns?

No. What you suggest looks as a neat and correct solution. I'll resend 
the updated patch after some testing (since now till Monday).

As for proof-of-correctness, all you wrote above is correct, but the 
first point had been boiling my mind for a while. I came to the 
following reasoning (hopefully it is what you meant):

The fact that filemap_write_and_wait() returned infers that 
end_page_writeback() was called for all relevant pages. And fuse doesn't 
call it before adding request to fi->queued_writes and calling 
fuse_flush_writepages(). And the latter, in turn, guarantees proper 
accounting of request in fi->writectr. Here, of course, it's crucial 
that we can't have concurrent fuse_set_nowrite(), as you explained. 
Hence, so far as fi->writectr was bumped, fuse_set_nowrite() we call 
after filemap_write_and_wait() would wait until all changes have gone to 
the server.

Thanks,
Maxim
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