linux-kernel - Re: [PATCH 1/2] mm: cma: fix allocation may fail sometimes

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Date:   Wed, 15 Dec 2021 13:30:52 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dong Aisheng <aisheng.dong@....com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, dongas86@...il.com,
        linux-arm-kernel@...ts.infradead.org, jason.hui.liu@....com,
        leoyang.li@....com, abel.vesa@....com, shawnguo@...nel.org,
        linux-imx@....com, akpm@...ux-foundation.org,
        m.szyprowski@...sung.com, lecopzer.chen@...iatek.com,
        vbabka@...e.cz, stable@...r.kernel.org, shijie.qin@....com
Subject: Re: [PATCH 1/2] mm: cma: fix allocation may fail sometimes

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@...+20@...+12@...+4@...+32@...+17@...7+23@...3+20@...76+99@...77+108@...52+44@...08+20@...96+108@...64+108@...20+
> 108@...00+108@...56+483@...61+1763@...41+1440@...12+20@...24+20@...88+5076@...52+2304@...40+35@...41+20@...20+20@...84+
> 7188@...48+84@...20+7276@...52+227@...25+6371@...49=> 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