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

From: Sergei Trofimovich
Date: Sat Jun 08 2013 - 02:13:33 EST

On Fri, 07 Jun 2013 23:47:33 -0400
Richard Yao <ryao@xxxxxxxxxx> 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?



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