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

[Aisheng Dong]

> 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.

Regards
Aisheng

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