linux-kernel - Re: [PATCH 1/1] FS: Add generic data flush to fsync

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Message-Id: <20140415190421.7dbf7ef08a545c6ab5538a5c@skynet.be>
Date:	Tue, 15 Apr 2014 19:04:21 +0200
From:	Fabian Frederick <fabf@...net.be>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Viro <viro@...iv.linux.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	jack <jack@...e.cz>, akpm <akpm@...ux-foundation.org>,
	tytso <tytso@....edu>
Subject: Re: [PATCH 1/1] FS: Add generic data flush to fsync

On Mon, 14 Apr 2014 00:22:42 -0700
Christoph Hellwig <hch@...radead.org> wrote:

> On Fri, Apr 11, 2014 at 11:05:30PM +0200, Fabian Frederick wrote:
> > Ok, that would mean 'older' filesystems -without barrier flag-
> > to call generic_file_fsync would result in a flush but ext4 without journal 
> > also calls it ; maybe with nobarrier and I'm not sure it's the only case.
> > How can I deal with this problem ?
> 
> I don't really think not implementing the barrier option is a problem.
> For ext4 if you want consistency either opencode generic_file_fsync
> there and add the check, or add a __generic_file_fsync that doesn't
> do the flush.

I've been searching once again for generic_file_fsync callsites and EXT4
is the only filesystem with barrier flag calling generic_file_fsync.

If Ted or Jan could tell me whether EXT4 without journal can issue the flush 
(blkdev_issue_flush) whatever option has been chosen for 'barrier' (as that option stands for journal barrier) -> simple case, just add blkdev_issue_flush.

or if it requires looking at barrier flag in ext4/fsync.c
In that case I'd be interested to know how (needs_barrier is based on journal
flags) ... 

-> generic_file_fsync calls __generic_file_fsync (flush=true)
-> ext4 calls __generic_file_fsync(false) if nobarrier.
 
Fabian
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