On Tue, Mar 08, 2016 at 09:42:00AM +0800, Xishi Qiu wrote:

On 2016/3/4 13:33, Hanjun Guo wrote:

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Hi Joonsoo,

On 2016/3/4 10:02, Joonsoo Kim wrote:

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On Thu, Mar 03, 2016 at 08:49:01PM +0800, Hanjun Guo wrote:

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On 2016/3/3 15:42, Joonsoo Kim wrote:

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2016-03-03 10:25 GMT+09:00 Laura Abbott [off-list ref]:

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(cc -mm and Joonsoo Kim)


On 03/02/2016 05:52 AM, Hanjun Guo wrote:

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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

[...]

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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.

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 - 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.

Thanks.

[A]

I tested this solution [A], it can fix the problem, as you are posting a new patch, I will
test that one and leave [B] alone :)

Hi Joonsoo,

How does this problem happen? Why the count is larger than total?

Patch A prevent the cma page free to pcp, right?

...
-               if (unlikely(is_migrate_isolate(migratetype))) {
+               if (is_migrate_cma(migratetype) ||
+                       unlikely(is_migrate_isolate(migratetype))) {
...

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.

Even without free to pcp, bad merging could happen. Please see another
thread I mentioned some example.

Thanks.