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Re: use-after-free on log replay failure

To: Alex Lyakas <alex@xxxxxxxxxxxxxxxxx>
Subject: Re: use-after-free on log replay failure
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 13 Aug 2014 10:03:12 +1000
Cc: Brian Foster <bfoster@xxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <DC407F6E8F8C4EE1AF7117D7D6ABF282@alyakaslap>
References: <4B2A412C75324EE9880358513C069476@alyakaslap> <9D3CBECB663B4A77B7EF74B67973310A@alyakaslap> <20140804230721.GA20518@dastard> <AC10852F403846A182491ED8071135ED@alyakaslap> <20140806152042.GB39990@xxxxxxxxxxxxxxx> <CAOcd+r3bC59m7Rh-3tmjrnWnF5XoPQfE=U+=hz78NcAGu+Ou1g@xxxxxxxxxxxxxx> <20140811132057.GA1186@xxxxxxxxxxxxxxx> <20140811215207.GS20518@dastard> <20140812120341.GA46654@xxxxxxxxxxxxxxx> <DC407F6E8F8C4EE1AF7117D7D6ABF282@alyakaslap>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Aug 12, 2014 at 03:39:02PM +0300, Alex Lyakas wrote:
> Then I set up the following Device Mapper target onto /dev/vde:
> dmsetup create VDE --table "0 41943040 linear-custom /dev/vde 0"
> I am attaching the code (and Makefile) of dm-linear-custom target.
> It is exact copy of dm-linear, except that it has a module
> parameter. With the parameter set to 0, this is an identity mapping
> onto /dev/vde. If the parameter is set to non-0, all WRITE bios are
> failed with ENOSPC. There is a workqueue to fail them in a different
> context (not sure if really needed, but that's what our "real"
> custom
> block device does).

FWIW, now I've looked at the dm module, this could easily be added
to the dm-flakey driver by adding a "queue_write_error" option
to it (i.e. similar to the current drop_writes and corrupt_bio_byte

If we add the code there, then we could add a debug-only XFS sysfs
variable to trigger the log recovery sleep, and then use dm-flakey
to queue and error out writes. That gives us a reproducable xfstest
for this condition. Brian, does that sound like a reasonable plan to

Thanks for describing the method you've been using to reproduce the
bug so clearly, Alex.


Dave Chinner

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