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

From: David Hildenbrand <redacted>
Sent: Wednesday, December 15, 2021 8:31 PM

On 15.12.21 09:02, Dong Aisheng wrote:

quoted

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@3607
6+99@40477+108

quoted

@40852+44@41108+20@41196+108@41364+108@41620+

108@42900+108@43156+483@44061+1763@45341+1440@47712+20@49
324+20@49388+

quoted

5076@49452+2304@55040+35@58141+20@58220+20@58284+
7188@58348+84@66220+7276@66452+227@74525+6371@75549=>

33161 free of

quoted

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

quoted

	    active_anon:98060kB inactive_anon:98948kB active_file:60864kB

inactive_file:31776kB

quoted

	    unevictable:0kB writepending:0kB present:1048576kB

managed:1018328kB mlocked:0kB

quoted

	    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)

quoted

	36*128kB (UMECI) 16*256kB (UMCI) 6*512kB (EI) 8*1024kB (UEI)

4*2048kB (MI) 8*4096kB (EI)

quoted

	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

quoted

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

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