[PATCH 1/2] mm: cma: fix allocation may fail sometimes

[David Hildenbrand]

On 17.12.21 04:44, Aisheng Dong wrote:
>> From: David Hildenbrand <david at redhat.com>
>> Sent: Thursday, December 16, 2021 6:57 PM
>>
>> On 16.12.21 03:54, Aisheng Dong wrote:
>>>> From: David Hildenbrand <david at redhat.com>
>>>> Sent: Wednesday, December 15, 2021 8:31 PM
>>>>
>>>> On 15.12.21 09:02, Dong Aisheng wrote:
>>>>> We met dma_alloc_coherent() fail sometimes when doing 8 VPU decoder
>>>>> test in parallel on a MX6Q SDB board.
>>>>>
>>>>> Error log:
>>>>> cma: cma_alloc: linux,cma: alloc failed, req-size: 148 pages, ret:
>>>>> -16
>>>>> cma: number of available pages:
>>>>>
>>>>
>> 3 at 125+20@172+12 at 236+4@380+32 at 736+17@2287+23 at 2473+20@3607
>>>> 6+99 at 40477+108
>>>>> @40852+44 at 41108+20@41196+108 at 41364+108@41620+
>>>>>
>>>>
>> 108 at 42900+108@43156+483 at 44061+1763@45341+1440 at 47712+20@49
>>>> 324+20 at 49388+
>>>>> 5076 at 49452+2304@55040+35 at 58141+20@58220+20 at 58284+
>>>>> 7188 at 58348+84@66220+7276 at 66452+227@74525+6371 at 75549=>
>>>> 33161 free of
>>>>> 81920 total pages
>>>>>
>>>>> When issue happened, we saw there were still 33161 pages (129M) free
>>>>> CMA memory and a lot available free slots for 148 pages in CMA
>>>>> bitmap that we want to allocate.
>>>>>
>>>>> If dumping memory info, we found that there was also ~342M normal
>>>>> memory, but only 1352K CMA memory left in buddy system while a lot
>>>>> of pageblocks were isolated.
>>>>>
>>>>> Memory info log:
>>>>> Normal free:351096kB min:30000kB low:37500kB high:45000kB
>>>> reserved_highatomic:0KB
>>>>> 	    active_anon:98060kB inactive_anon:98948kB active_file:60864kB
>>>> inactive_file:31776kB
>>>>> 	    unevictable:0kB writepending:0kB present:1048576kB
>>>> managed:1018328kB mlocked:0kB
>>>>> 	    bounce:0kB free_pcp:220kB local_pcp:192kB free_cma:1352kB
>>>>> lowmem_reserve[]: 0 0 0
>>>>> Normal: 78*4kB (UECI) 1772*8kB (UMECI) 1335*16kB (UMECI) 360*32kB
>>>> (UMECI) 65*64kB (UMCI)
>>>>> 	36*128kB (UMECI) 16*256kB (UMCI) 6*512kB (EI) 8*1024kB (UEI)
>>>> 4*2048kB (MI) 8*4096kB (EI)
>>>>> 	8*8192kB (UI) 3*16384kB (EI) 8*32768kB (M) = 489288kB
>>>>>
>>>>> The root cause of this issue is that since commit a4efc174b382
>>>>> ("mm/cma.c: remove redundant cma_mutex lock"), CMA supports
>>>> concurrent
>>>>> memory allocation. It's possible that the pageblock process A try to
>>>>> alloc has already been isolated by the allocation of process B
>>>>> during memory migration.
>>>>>
>>>>> When there're multi process allocating CMA memory in parallel, it's
>>>>> likely that other the remain pageblocks may have also been isolated,
>>>>> then CMA alloc fail finally during the first round of scanning of
>>>>> the whole available CMA bitmap.
>>>>
>>>> I already raised in different context that we should most probably
>>>> convert that -EBUSY to -EAGAIN --  to differentiate an actual
>>>> migration problem from a simple "concurrent allocations that target the
>> same MAX_ORDER -1 range".
>>>>
>>>
>>> Thanks for the info. Is there a patch under review?
>>
>> No, and I was too busy for now to send it out.
>>
>>> BTW i wonder that probably makes no much difference for my patch since
>>> we may prefer retry the next pageblock rather than busy waiting on the
>> same isolated pageblock.
>>
>> Makes sense. BUT as of now we isolate not only a pageblock but a
>> MAX_ORDER -1 page (e.g., 2 pageblocks on x86-64 (!) ). So you'll have the
>> same issue in that case.
> 
> Yes, should I change to try next MAX_ORDER_NR_PAGES or keep as it is
> and let the core to improve it later?
> 
> I saw there's a patchset under review which is going to remove the
> MAX_ORDER - 1 alignment requirement for CMA.
> https://patchwork.kernel.org/project/linux-mm/cover/20211209230414.2766515-1-zi.yan@sent.com/
> 
> Once it's merged, I guess we can back to align with pageblock rather
> than MAX_ORDER-1.

While the goal is to get rid of the alignment requirement, we might
still have to isolate all applicable MAX_ORDER-1 pageblocks. Depends on
what we can or cannot achieve easily :)


-- 
Thanks,

David / dhildenb