Re: [PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc_coherent() calls

Soeren Moch
2013-01-16T03:25:18Z

On 16.01.2013 03:40, Jason Cooper wrote:
> Soeren,
>
> On Wed, Jan 16, 2013 at 01:17:59AM +0100, Soeren Moch wrote:
>> On 15.01.2013 22:56, Jason Cooper wrote:
>>> On Tue, Jan 15, 2013 at 03:16:17PM -0500, Jason Cooper wrote:
>>>> If my understanding is correct, one of the drivers (most likely one)
>>>> either asks for too small of a dma buffer, or is not properly
>>>> deallocating blocks from the per-device pool. Either case leads to
>>>> exhaustion, and falling back to the atomic pool. Which subsequently
>>>> gets wiped out as well.
>>>
>>> If my hunch is right, could you please try each of the three dvb drivers
>>> in turn and see which one (or more than one) causes the error?
>>
>> In fact I use only 2 types of DVB sticks: em28xx usb bridge plus drxk
>> demodulator, and dib0700 usb bridge plus dib7000p demod.
>>
>> I would bet for em28xx causing the error, but this is not thoroughly
>> tested. Unfortunately testing with removed sticks is not easy, because
>> this is a production system and disabling some services for the long
>> time we need to trigger this error will certainly result in unhappy
>> users.
>
> Just out of curiosity, what board is it?

The kirkwood board? A modified Guruplug Server Plus.
>
>> I will see what I can do here. Is there an easy way to track the buffer
>> usage without having to wait for complete exhaustion?
>
> DMA_API_DEBUG

OK, maybe I can try this.
>
>> In linux-3.5.x there is no such problem. Can we use all available memory
>> for dma buffers here on armv5 architectures, in contrast to newer
>> kernels?
>
> Were the loads exactly the same when you tested 3.5.x?

Exactly the same, yes.

>I looked at the
> changes from v3.5 to v3.7.1 for all four drivers you mentioned as well
> as sata_mv.
>
> The biggest thing I see is that all of the media drivers got shuffled
> around into their own subdirectories after v3.5. 'git show -M 0c0d06c'
> shows it was a clean copy of all the files.
>
> What would be most helpful is if you could do a git bisect between
> v3.5.x (working) and the oldest version where you know it started
> failing (v3.7.1 or earlier if you know it).
>
I did not bisect it, but Marek mentioned earlier that commit
e9da6e9905e639b0f842a244bc770b48ad0523e9 in Linux v3.6-rc1 introduced
new code for dma allocations. This is probably the root cause for the
new (mis-)behavior (due to my tests 3.6.0 is not working anymore).
I'm not very familiar with arm mm code, and from the patch itself I
cannot understand what's different. Maybe CONFIG_CMA is default
also for armv5 (not only v6) now? But I might be totally wrong here,
maybe someone of the mm experts can explain the difference?

Regards,
Soeren