linux-kernel - Re: [PATCH 0/4] Fix a crash when block device is read and block size is changed at the same time

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Date:	Tue, 25 Sep 2012 13:49:51 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jens Axboe <axboe@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Jan Kara <jack@...e.cz>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	kosaki.motohiro@...fujitsu.com, linux-fsdevel@...r.kernel.org,
	lwoodman@...hat.com, "Alasdair G. Kergon" <agk@...hat.com>
Subject: Re: [PATCH 0/4] Fix a crash when block device is read and block size is changed at the same time

Jeff Moyer <jmoyer@...hat.com> writes:

> Mikulas Patocka <mpatocka@...hat.com> writes:
>
>> Hi Jeff
>>
>> Thanks for testing.
>>
>> It would be interesting ... what happens if you take the patch 3, leave 
>> "struct percpu_rw_semaphore bd_block_size_semaphore" in "struct 
>> block_device", but remove any use of the semaphore from fs/block_dev.c? - 
>> will the performance be like unpatched kernel or like patch 3? It could be 
>> that the change in the alignment affects performance on your CPU too, just 
>> differently than on my CPU.
>
> It turns out to be exactly the same performance as with the 3rd patch
> applied, so I guess it does have something to do with cache alignment.
> Here is the patch (against vanilla) I ended up testing.  Let me know if
> I've botched it somehow.
>
> So, I next up I'll play similar tricks to what you did (padding struct
> block_device in all kernels) to eliminate the differences due to
> structure alignment and provide a clear picture of what the locking
> effects are.

After trying again with the same padding you used in the struct
bdev_inode, I see no performance differences between any of the
patches.  I tried bumping up the number of threads to saturate the
number of cpus on a single NUMA node on my hardware, but that resulted
in lower IOPS to the device, and hence consumption of less CPU time.
So, I believe my results to be inconclusive.

After talking with Vivek about the problem, he had mentioned that it
might be worth investigating whether bd_block_size could be protected
using SRCU.  I looked into it, and the one thing I couldn't reconcile is
updating both the bd_block_size and the inode->i_blkbits at the same
time.  It would involve (afaiui) adding fields to both the inode and the
block_device data structures and using rcu_assign_pointer  and
rcu_dereference to modify and access the fields, and both fields would
need to protected by the same struct srcu_struct.  I'm not sure whether
that's a desirable approach.  When I started to implement it, it got
ugly pretty quickly.  What do others think?

For now, my preference is to get the full patch set in.  I will continue
to investigate the performance impact of the data structure size changes
that I've been seeing.

So, for the four patches:

Acked-by: Jeff Moyer <jmoyer@...hat.com>

Jens, can you have a look at the patch set?  We are seeing problem
reports of this in the wild[1][2].

Cheers,
Jeff

[1] https://bugzilla.redhat.com/show_bug.cgi?id=824107
[2] https://bugzilla.redhat.com/show_bug.cgi?id=812129
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