AW: Possible RAID6 regression with ASYNC_TX_DMA enabled in 4.1

[Markus Stockhausen]

Hi Maxime,

> Von: linux-raid-owner@xxxxxxxxxxxxxxx [linux-raid-owner@xxxxxxxxxxxxxxx]" im Auftrag von "Maxime Ripard [maxime.ripard@xxxxxxxxxxxxxxxxxx]
> Gesendet: Donnerstag, 7. Mai 2015 14:57
> An: Neil Brown; Shaohua Li
> Cc: linux-raid@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Lior Amsalem; Thomas Petazzoni; Gregory Clement; Boris Brezillon
> Betreff: Possible RAID6 regression with ASYNC_TX_DMA enabled in 4.1
> 
> Hi,
> 
> I'm currently trying to add support for the PQ operations on the
> marvell XOR engine, in dmaengine, obviously to be able to use async_tx
> to offload these operations.
> 
> I'm testing these patches with a RAID6 array with 4 disks.
> 
> However, since the commit 59fc630b8b5f ("RAID5: batch adjacent full
> stripe write", every write to that array fails with the following
> stacktrace.
> 
> http://code.bulix.org/eh8iew-88342?raw

I don't know if it might be related. I added support for RAID6 Read-Modify-Write
in software XOR with some patches. The following commit mangles some lines in 
async_pq.c:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?
id=584acdd49cd2472ca0f5a06adbe979db82d0b4af

I introduced a new flag ASYNC_TX_PQ_XOR_DST that notifies the async layer
that we want to do a XOR syndrome operation instead of a full calculation.
This will enforce the software path because I guessed that hardware does not
support that case. Without hardware to check I might have missed some 
checks in the async layer.

In the upper layer ops_run_reconstruct6 will set the flag if we determined
that rmw is faster than rcw.

Can you check if rmw_level=0 fixes the issue. See:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?
id=d06f191f8ecaef4d524e765fdb455f96392fbd42

> It seems to be generated by that warning here:
> 
> http://lxr.free-electrons.com/source/crypto/async_tx/async_tx.c#L173
> 
> And indeed, if we dump the status of depend_tx here, it's already been
> acked.
> 
> That doesn't happen if ASYNC_TX_DMA is disabled, hence using the
> software version of it, instead of relying on our XOR engine. It
> doesn't happen on any commit prior to the one mentionned above, with
> the exact same changes applied. These changes are meant to be
> contributed, so I can definitely push them somewhere if needed.
> 
> I don't really know where to look for though, the change that is
> causing this is probably the change in ops_run_reconstruct6, but I'm
> not sure that this partial revert alone would work with regard to the
> rest of the patch.
> 
> Maxime

Markus
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