Re: [GIT PULL] Core block IO bits for 2.6.39 - early Oops

[Jens Axboe]

On 2011-03-25 12:59, Theodore Tso wrote:
> 
> On Mar 25, 2011, at 12:41 AM, Dave Chinner wrote:
> 
>>>
>>> It works insofar as the Oops is gone. But my xfs partitions apparently
>>> still get corrupted (I had to run xfs_repair on several of them, because
>>> they would not mount otherwise).
>>
>> So the patchset is causing repeatable filesystem corruption? Sounds
>> to me like this series is not yet ready for mainline merging. Last
>> thing I want to spend the .39 cycle helping people recover busted
>> filesystems as a result of undercooked block layer changes...
> 
> FYI.   I did a trial merge last night of the ext4 changes last night with
> the tip of Linus's tree.   The ext4 changes (based on 2.6.38-rc5) 
> survived xfstests -g auto before I merged in Linus's 2.6.39 master
> branch.  After I merged with 2.6.39-tip, I reran xfstests, and it got 
> past test #13 (fsstress), which normally means that everything is
> OK, so I sent a pull request to Linus.    Much later, (-g auto takes a 
> long time) I got an OOPS inside the virtio driver.   Ext4 was nowhere 
> in the stack trace, but of course the block layer was.   Grumbling
> that someone  had broke virtio during the merge window, I switched
> my KVM setup to use SATA emulation and used the sda devices
> instead.  This time I got an oops in the block I/O layer, again quite
> late in xfstests.  Somewhere around test #224 or so if I remember
> correctly.
> 
> It was too late last night to do any more investigating, which is why
> I hadn't sent a formal report yet, but next up is for me to retry xfstests
> before merging in my changes, and then to start a git bisect.
> 
> So before accusing some patch series which hasn't been merged
> into 2.6.39 yet, you might want to also worry about some change
> that already has been merged.   Of course the symptoms for me are
> quite different.   I'm not seeing an early oops, but only something
> which shows up when the the system is put under a lot of stress
> by xfstests.  So it could be a different problem....
> 
> 								- Ted
> 
> P.S.  And of course there is the chance that there is some
> subtle bug in the ext4 branch, which worked just fine when
> it was just based on 2.6.38-rc5, but which only manifested
> itself when I merged in the tip of Linus's branch.   So I'm not
> __accusing__ the block layer yet, even though the stack traces
> seem to point that way, because I don't have a smoking gun
> yet.   But I do have to admit I'm suspicious....

But this plugging change is merged, so it is a very likely candidate.
With the oddness going on, I suspect that we end up flushing a plug that
resides on a stack that is no longer valid.

Is there a way to check whether a given pointer is valid on the current
stack for this process?

I think we can rule out stack overflows, since the plug context itself
is very small (28 bytes). But if we have something like:

blk_start_plug(&plug1);
        ...
        blk_start_plug(&plug2);
        ...
flush(&plug2);

then that could explain the corruption and lockups.

So I'd really like to have something ala:

        if (is_str_ptr_valid(current, ptr, size))
                ...

to aid the debugging.

-- 
Jens Axboe

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