linux-kernel - Re: [PATCH 1/1] fs/mpage.c: forgotten WRITE

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Message-ID: <20140314141143.GH12613@htj.dyndns.org>
Date:	Fri, 14 Mar 2014 10:11:43 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Roman Peniaev <r.peniaev@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 1/1] fs/mpage.c: forgotten WRITE_SYNC in case of data
 integrity write

Hello,

On Fri, Mar 14, 2014 at 11:07:04PM +0900, Roman Peniaev wrote:
> Seems the following message should be better:
> When data inegrity operation (sync, fsync, fdatasync calls) happens
> writeback control is set to WB_SYNC_ALL.
> In that case all write requests are marked with WRITE_SYNC, but on
> mpage writeback path
> WRITE_SYNC is missed. This patch fixes this.
> 
> Is it ok, what do you think?

I think the description should make it clear that WRITE_SYNC is about
latency, not about integrity and we probably should add comments
explaining why we're using WRITE_SYNC for WB_SYNC_ALL (because there
probably is someone waiting).

> Also, could you please help me do understand how can I guarantee
> integrity in case of block device with big volatile
> cache and filesystem, which does not support REQ_FLUSH/FUA?

If a device has a volatile cache but doesn't support flush, it can't
guarantee integrity.  There's no way for its user to determine or
force whether certain data is on non-volatile media.  It's an
inherently broken device.

Thanks.

-- 
tejun
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