linux-kernel - Re: [gentoo-dev] dm-crypt reordering BIOs across barriers?

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Message-ID: <20130608091114.441c5f17@sf>
Date:	Sat, 8 Jun 2013 09:11:14 +0300
From:	Sergei Trofimovich <slyfox@...too.org>
To:	gentoo-dev@...ts.gentoo.org
Cc:	ryao@...too.org,
	Kernel development list <linux-kernel@...r.kernel.org>,
	kernel@...too.org, gentooligan@...il.com
Subject: Re: [gentoo-dev] dm-crypt reordering BIOs across barriers?

On Fri, 07 Jun 2013 23:47:33 -0400
Richard Yao <ryao@...too.org> wrote:

> When you use dm-crypt, block IO requests to a dm-* device will invoke
> dm_request_fn() -> map_request() -> crypt_map(). If a BIO is a write
> barrier, crypt_map() will return DM_MAPIO_REMAPPED to map_request(),
> which will immediately queue it to the device.
> 
> If a few dozen IOs are queued in rapid succession with multiple write
> barriers, all write barriers will be executed before any actual write
> BIOs occur because the write IOs will be processed asynchronously in a
> work queue. Since the barriers will be long gone by the time the write
> IOs are queued, they can be queued in any order.
> 
> Am I misunderstanding this or is dm-crypt ignoring proper write barrier
> semantics?
> 
http://www.saout.de/pipermail/dm-crypt/2012-April/002441.html
http://lwn.net/Articles/400541/

-- 

  Sergei

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