linux-kernel - Re: [PATCH v3 1/2] mm: cma: fix allocation may fail sometimes

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Date:   Thu, 17 Mar 2022 11:55:39 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dong Aisheng <aisheng.dong@....com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        dongas86@...il.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 v3 1/2] mm: cma: fix allocation may fail sometimes

On 15.03.22 15:45, Dong Aisheng wrote:
> When there're multiple process allocing dma memory in parallel

s/allocing/allocating/

> by calling dma_alloc_coherent(), it may fail sometimes as follows:
> 
> 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.

s/If/When/

> 
> 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 memory range process A trying
> to alloc has already been isolated by the allocation of process B during
> memory migration.
> 
> The problem here is that the memory range isolated during one allocation
> by start_isolate_page_range() could be much bigger than the real size we
> want to alloc due to the range is aligned to MAX_ORDER_NR_PAGES.
> 
> Taking an ARMv7 platform with 1G memory as an example, when MAX_ORDER_NR_PAGES
> is big (e.g. 32M with max_order 14) and CMA memory is relatively small
> (e.g. 128M), there're only 4 MAX_ORDER slot, then it's very easy that
> all CMA memory may have already been isolated by other processes when
> one trying to allocate memory using dma_alloc_coherent().
> Since current CMA code will only scan one time of whole available CMA
> memory, then dma_alloc_coherent() may easy fail due to contention with
> other processes.
> 
> This patch introduces a retry mechanism to rescan CMA bitmap for -EBUSY
> error in case the target memory range may has been temporarily isolated
> by others and released later.

But you patch doesn't check for -EBUSY and instead might retry forever,
on any allocation error, no?

I'd really suggest letting alloc_contig_range() return -EAGAIN in case
the isolation failed and handling -EAGAIN only in a special way instead.

In addition, we might want to stop once we looped to often I assume.

-- 
Thanks,

David / dhildenb