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From:   Zi Yan <ziy@nvidia.com>
To:     Qian Cai <quic_qiancai@quicinc.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Eric Ren <renzhengeek@gmail.com>,
        Mike Rapoport <rppt@kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v11 0/6] Use pageblock_order for cma and alloc_contig_range alignment.
Date:   Thu, 19 May 2022 19:24:48 -0400
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On 19 May 2022, at 17:35, Zi Yan wrote:

> On 19 May 2022, at 16:57, Qian Cai wrote:
>
>> On Thu, Apr 28, 2022 at 08:39:06AM -0400, Zi Yan wrote:
>>> How about the one attached? I can apply it to next-20220428. Let me k=
now
>>> if you are using a different branch. Thanks.
>>
>> Zi, it turns out that the endless loop in isolate_single_pageblock() c=
an
>> still be reproduced on today's linux-next tree by running the reproduc=
er a
>> few times. With this debug patch applied, it keeps printing the same
>> values.
>>
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -399,6 +399,8 @@ static int isolate_single_pageblock(unsigned long =
boundary_pfn, gfp_t gfp_flags,
>>                                 };
>>                                 INIT_LIST_HEAD(&cc.migratepages);
>>
>> +                               printk_ratelimited("KK stucked pfn=3D%=
lu head_pfn=3D%lu nr_pages=3D%lu boundary_pfn=3D%lu\n", pfn, head_pfn, nr=
_pages, boundary_pfn);
>>                                 ret =3D __alloc_contig_migrate_range(&=
cc, head_pfn,
>>                                                         head_pfn + nr_=
pages);
>>
>>  isolate_single_pageblock: 179 callbacks suppressed
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>>  KK stucked pfn=3D2151120384 head_pfn=3D2151120384 nr_pages=3D512 boun=
dary_pfn=3D2151120896
>
> Hi Qian,
>
> Thanks for your testing.
>
> 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 th=
e 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 repr=
oduce locally to
> identify what makes __alloc_contig_migrate_range() return 0 without mig=
rating the page.
>
> Can you also try the patch below to see if it fixes the infinite loop?

I also have an off-by-one error in the code. The error caused unnecessary=
 effort of
trying to migrate some pages. Your endless loop case seems to be caused b=
y it.
Can you actually try the patch below? Thanks.

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index b3f074d1682e..5c8099bb822f 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -374,7 +374,7 @@ static int isolate_single_pageblock(unsigned long bou=
ndary_pfn, gfp_t gfp_flags,
                        struct page *head =3D compound_head(page);
                        unsigned long head_pfn =3D page_to_pfn(head);

-                       if (head_pfn + nr_pages < boundary_pfn) {
+                       if (head_pfn + nr_pages <=3D boundary_pfn) {
                                pfn =3D head_pfn + nr_pages;
                                continue;
                        }
@@ -417,10 +417,8 @@ static int isolate_single_pageblock(unsigned long bo=
undary_pfn, gfp_t gfp_flags,
                                order =3D 0;
                                outer_pfn =3D pfn;
                                while (!PageBuddy(pfn_to_page(outer_pfn))=
) {
-                                       if (++order >=3D MAX_ORDER) {
-                                               outer_pfn =3D pfn;
-                                               break;
-                                       }
+                                       if (++order >=3D MAX_ORDER)
+                                               goto failed;
                                        outer_pfn &=3D ~0UL << order;
                                }
                                pfn =3D outer_pfn;

--
Best Regards,
Yan, Zi

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