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

[Aisheng Dong]

> 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?
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.
Otherwise, we may meet the same issue as the patch 2/2 wants to address.

How do you think?

Regards
Aisheng

> 
> --
> Thanks,
> 
> David / dhildenb