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@125+20@172+12@236+4@380+32@736+17@2287+23@2473+20@36076+99@40477+108@40852+44@41108+20@41196+108@41364+108@41620+
> 108@42900+108@43156+483@44061+1763@45341+1440@47712+20@49324+20@49388+5076@49452+2304@55040+35@58141+20@58220+20@58284+
> 7188@58348+84@66220+7276@66452+227@74525+6371@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,
David / dhildenb
