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

[Minchan Kim <minchan@kernel.org>]

On Thu, Mar 17, 2022 at 10:26:42PM +0800, Dong Aisheng wrote:
> On Thu, Mar 17, 2022 at 6:55 PM David Hildenbrand <david@redhat.com> 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.

Yes, I also have met the problem especially when the multiple threads
compete cma allocation. Thanks for bringing up the issue.

> > >
> > > 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.

IMO, when the CMA see the -EAGAIN, it should put the task into
cma->wait_queue and then be woken up by other thread which finish
work of the cma. So it's similar with cma_mutex but we don't need to
synchronize for !EAGAIN cases and make the cma allocatoin fair.