Re: Suspicious error for CMA stress test

[Joonsoo Kim]

On Fri, Mar 04, 2016 at 11:02:33AM +0900, Joonsoo Kim wrote:
> On Thu, Mar 03, 2016 at 08:49:01PM +0800, Hanjun Guo wrote:
> > On 2016/3/3 15:42, Joonsoo Kim wrote:
> > > 2016-03-03 10:25 GMT+09:00 Laura Abbott <labbott@xxxxxxxxxx>:
> > >> (cc -mm and Joonsoo Kim)
> > >>
> > >>
> > >> On 03/02/2016 05:52 AM, Hanjun Guo wrote:
> > >>> Hi,
> > >>>
> > >>> I came across a suspicious error for CMA stress test:
> > >>>
> > >>> Before the test, I got:
> > >>> -bash-4.3# cat /proc/meminfo | grep Cma
> > >>> CmaTotal:         204800 kB
> > >>> CmaFree:          195044 kB
> > >>>
> > >>>
> > >>> After running the test:
> > >>> -bash-4.3# cat /proc/meminfo | grep Cma
> > >>> CmaTotal:         204800 kB
> > >>> CmaFree:         6602584 kB
> > >>>
> > >>> So the freed CMA memory is more than total..
> > >>>
> > >>> Also the the MemFree is more than mem total:
> > >>>
> > >>> -bash-4.3# cat /proc/meminfo
> > >>> MemTotal:       16342016 kB
> > >>> MemFree:        22367268 kB
> > >>> MemAvailable:   22370528 kB
> > [...]
> > >>
> > >> I played with this a bit and can see the same problem. The sanity
> > >> check of CmaFree < CmaTotal generally triggers in
> > >> __move_zone_freepage_state in unset_migratetype_isolate.
> > >> This also seems to be present as far back as v4.0 which was the
> > >> first version to have the updated accounting from Joonsoo.
> > >> Were there known limitations with the new freepage accounting,
> > >> Joonsoo?
> > > I don't know. I also played with this and looks like there is
> > > accounting problem, however, for my case, number of free page is slightly less
> > > than total. I will take a look.
> > >
> > > Hanjun, could you tell me your malloc_size? I tested with 1 and it doesn't
> > > look like your case.
> > 
> > I tested with malloc_size with 2M, and it grows much bigger than 1M, also I
> > did some other test:
> 
> Thanks! Now, I can re-generate erronous situation you mentioned.
> 
> > 
> >  - run with single thread with 100000 times, everything is fine.
> > 
> >  - I hack the cam_alloc() and free as below [1] to see if it's lock issue, with
> >    the same test with 100 multi-thread, then I got:
> 
> [1] would not be sufficient to close this race.
> 
> Try following things [A]. And, for more accurate test, I changed code a bit more
> to prevent kernel page allocation from cma area [B]. This will prevent kernel
> page allocation from cma area completely so we can focus cma_alloc/release race.
> 
> Although, this is not correct fix, it could help that we can guess
> where the problem is.

More correct fix is something like below.
Please test it.

It checks problematic buddy merging and prevent it.
I will try to find another way that is less intrusive for freepath performance.

Thanks.

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