block layer bug with 4.4-rc3+

[Ming Lei]

On Tue, Dec 15, 2015 at 7:05 PM, Andre Przywara <andre.przywara at arm.com> wrote:
> Hi,
>
> I've been experiencing issues with at least 4.4-rc3 (including current

I'd suggest you to test the latest linus tree first, and at least two
fix patches
have been merged for blk-merge issue.  If there is still the issue
with linus tree,
I am happy to take a look.

Thanks,

> HEAD) on a Calxeda Midway (4*ARM Cortex-A15 (32-bit), 8GB RAM, SATA
> spinning disk or SSD).
> After some disk I/O load (kernel compile with -j6) I see the kernel
> screaming:
>
> [  103.736982] ata1.00: exception Emask 0x0 SAct 0x3ffff0 SErr 0x0
> action 0x6 frozen
> [  103.744476] ata1.00: failed command: WRITE FPDMA QUEUED
> [  103.749707] ata1.00: cmd 61/00:20:48:6b:41/08:00:0a:00:00/40 tag 4
> ncq 1048576 out
> [  103.749707]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask
> 0x4 (timeout)
> [  103.764659] ata1.00: status: { DRDY }
> [  103.768321] ata1.00: failed command: WRITE FPDMA QUEUED
> [  103.773547] ata1.00: cmd 61/98:28:48:73:41/42:00:0a:00:00/40 tag 5
> ncq 8728576 out
> [  103.773547]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask
> 0x4 (timeout)
> < repeated with increasing tag numbers>
>
> This repeats for a while, but then seems to recover later, though I
> haven't checked if there are more issues and rebooted instead to avoid
> filesystem damage.
>
> While I agree that this looks like a disk error on the first glance, I
> never saw this before 4.4-rc2, had the very same error on different
> nodes (with another spinning disk and even an SSD) and I can make it
> vanish by reverting the commit I identified after bisection:
>
> commit 578270bfbd2803dc7b0b03fbc2ac119efbc73195
> Author: Ming Lei <ming.lei at canonical.com>
> Date:   Tue Nov 24 10:35:29 2015 +0800
>
>     block: fix segment split
> ...
> I understand that this fix seems sane, but actually reverting it fixes
> the issue for me: 4.4-rc5 crashed within some minutes with the above
> log, 4.4-rc5 with 578270bfbd reverted survived 19 hours of continuous
> kernel compiles without issues.
> Looking at the git history of that file I see quite some recent changes
> there, but it's beyond my understanding of the code to spot the real
> culprit.
>
> Can anyone point me to a change in blk-merge.c I could try to revert to
> identify the real root cause? I can run tests quickly, though a real
> positive case would need some hours of runtime to be sure it's fine.
>
> Many thanks!
> Cheers,
> Andre.
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