Re: Suspicious error for CMA stress test

[Hanjun Guo]

On 2016/3/4 10:02, 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.
>
> Thanks.
>
> [A]
> diff --git a/mm/cma.c b/mm/cma.c
> index c003274..43ed02d 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -496,7 +496,9 @@ bool cma_release(struct cma *cma, const struct page *pages, unsigned int count)
>  
>         VM_BUG_ON(pfn + count > cma->base_pfn + cma->count);
>  
> +       mutex_lock(&cma_mutex);
>         free_contig_range(pfn, count);
> +       mutex_unlock(&cma_mutex);
>         cma_clear_bitmap(cma, pfn, count);
>         trace_cma_release(pfn, pages, count);
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c6c38ed..1ce8a59 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2192,7 +2192,8 @@ void free_hot_cold_page(struct page *page, bool cold)
>          * excessively into the page allocator
>          */
>         if (migratetype >= MIGRATE_PCPTYPES) {
> -               if (unlikely(is_migrate_isolate(migratetype))) {
> +               if (is_migrate_cma(migratetype) ||
> +                       unlikely(is_migrate_isolate(migratetype))) {
>                         free_one_page(zone, page, pfn, 0, migratetype);
>                         goto out;
>                 }

As I replied in previous email, the solution will fix the problem, the Cma freed memory and
system freed memory is in sane state after apply above patch.

I also tested this situation which only apply the code below:

        if (migratetype >= MIGRATE_PCPTYPES) {
-               if (unlikely(is_migrate_isolate(migratetype))) {
+               if (is_migrate_cma(migratetype) ||
+                       unlikely(is_migrate_isolate(migratetype))) {
                        free_one_page(zone, page, pfn, 0, migratetype);
                        goto out;
                }


This will not fix the problem, but will reduce the errorous freed number of memory,
hope this helps.

>
>
> [B]
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f2dccf9..c6c38ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1493,6 +1493,7 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>                                                                 int alloc_flags)
>  {
>         int i;
> +       bool cma = false;
>  
>         for (i = 0; i < (1 << order); i++) {
>                 struct page *p = page + i;
> @@ -1500,6 +1501,9 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>                         return 1;
>         }
>  
> +       if (is_migrate_cma(get_pcppage_migratetype(page)))
> +               cma = true;
> +
>         set_page_private(page, 0);
>         set_page_refcounted(page);
>  
> @@ -1528,6 +1532,12 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>         else
>                 clear_page_pfmemalloc(page);
>  
> +       if (cma) {
> +               page_ref_dec(page);

mm/page_alloc.c: In function ‘prep_new_page’:
mm/page_alloc.c:1407:3: error: implicit declaration of function ‘page_ref_dec’ [-Werror=implicit-function-declaration]
   page_ref_dec(page);
   ^

Typo?

Thanks
Hanjun