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

[Dong Aisheng]

On Thu, Mar 17, 2022 at 6:55 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> 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@125+20@172+12@236+4@380+32@736+17@2287+23@2473+20@36076+99@40477+108@40852+44@41108+20@41196+108@41364+108@41620+
> > 108@42900+108@43156+483@44061+1763@45341+1440@47712+20@49324+20@49388+5076@49452+2304@55040+35@58141+20@58220+20@58284+
> > 7188@58348+84@66220+7276@66452+227@74525+6371@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.
>
> s/If/When/
>

Will fix them all, thanks.

> >
> > 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?
>

My patch seems not need check it because there's no chance to retry the loop
in case an non -EBUS error happened earlier.

for (;;) {
        if (bitmap_no >= bitmap_maxno) {
                retry_the_whole_loop;
        }

        pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
        ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
                             GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0));

        if (ret != -EBUSY)
                break;
}

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

Yes, i guess that's another improvement and is applicable.

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

I wonder if really retried un-reasonably too often, we probably may
need figure out
what's going on inside alloc_contig_range() and fix it rather than
return EBUSY error to
users in case there're still a lot of avaiable memories.
So currently i didn't add a maximum retry loop outside.

Additionaly, for a small CMA system (128M with 32M max_order pages),
the retry would
be frequently when multiple process allocating memory, it also depends
on system running
state, so it's hard to define a reasonable and stable maxinum retry count.

Regards
Aisheng

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