linux-kernel - Re: [PATCH v11 0/6] Use pageblock_order for cma and alloc_contig

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Date:   Fri, 20 May 2022 10:13:51 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Qian Cai <quic_qiancai@...cinc.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Eric Ren <renzhengeek@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v11 0/6] Use pageblock_order for cma and alloc_contig_range alignment.

On 20 May 2022, at 9:43, Zi Yan wrote:

> On 20 May 2022, at 7:30, Qian Cai wrote:
>
>> On Thu, May 19, 2022 at 05:35:15PM -0400, Zi Yan wrote:
>>> Do you have a complete reproducer? From your printout, it is clear that a 512-page compound
>>> page caused the infinite loop, because the page was not migrated and the code kept
>>> retrying. But __alloc_contig_migrate_range() is supposed to return non-zero to tell the
>>> code the page cannot be migrated and the code will goto failed without retrying. It will be
>>> great you can share what exactly has run after boot, so that I can reproduce locally to
>>> identify what makes __alloc_contig_migrate_range() return 0 without migrating the page.
>>
>> The reproducer is just to run the same script I shared with you previously
>> multiple times instead. It is still quite reproducible here as it usually
>> happens within a hour.
>>
>> $ for i in `seq 1 100`; do ./flip_mem.py; done

Also, do you mind providing the page dump of the 512-page compound page? I would like
to know what page caused the issue.

Thanks.

>>
>>> Can you also try the patch below to see if it fixes the infinite loop?
>>>
>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>> index b3f074d1682e..abde1877bbcb 100644
>>> --- a/mm/page_isolation.c
>>> +++ b/mm/page_isolation.c
>>> @@ -417,10 +417,9 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
>>>                                 order = 0;
>>>                                 outer_pfn = pfn;
>>>                                 while (!PageBuddy(pfn_to_page(outer_pfn))) {
>>> -                                       if (++order >= MAX_ORDER) {
>>> -                                               outer_pfn = pfn;
>>> -                                               break;
>>> -                                       }
>>> +                                       /* abort if the free page cannot be found */
>>> +                                       if (++order >= MAX_ORDER)
>>> +                                               goto failed;
>>>                                         outer_pfn &= ~0UL << order;
>>>                                 }
>>>                                 pfn = outer_pfn;
>>>
>>
>> Can you explain a bit how this patch is the right thing to do here? I am a
>> little bit worry about shooting into the dark. Otherwise, I'll be running
>> the off-by-one part over the weekend to see if that helps.
>
> The code kept retrying to migrate a 512-page compound page, so it seems to me
> that __alloc_contig_migrate_range() did not migrate the page but returned
> 0 every time, otherwise, if (ret) goto failed; would bail out of the loop
> already. The original code above assumed a free page can always be found after
> __alloc_contig_migrate_range(), so it will retry if no free page is found.
> But that assumption is not true from your infinite loop result, the new
> code quits retrying when no free page can be found.
>
> I will dig into it deeper to make sure it is the correct fix. I will
> update you when I am done.
>
> Thanks.
>
> --
> Best Regards,
> Yan, Zi


--
Best Regards,
Yan, Zi

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