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

Jeff Moyer <jmoyer redhat com> writes:

> Mikulas Patocka <mpatocka redhat 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 redhat com>

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


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

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