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

[Dong Aisheng]

On Fri, Dec 17, 2021 at 8:27 PM David Hildenbrand <david at redhat.com> wrote:
>
> On 17.12.21 04:44, Aisheng Dong wrote:
> >> 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.
>
> While the goal is to get rid of the alignment requirement, we might
> still have to isolate all applicable MAX_ORDER-1 pageblocks. Depends on
> what we can or cannot achieve easily :)
>

Ok, got it. As that's another story and does not affect us to fix the current
kernel problem first that CMA alloc may fail occasionally,
I'm going to change to align with MAX_ORDER_NR_PAGES for retries
as you pointed out in the next version.
Do you have more suggestions for this patchset?

Regards
Aisheng

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