Re: [PATCH] mm/compaction: count pages and stop correctly during page isolation.

[Zi Yan]

On 29 Oct 2020, at 17:14, Yang Shi wrote:

> On Thu, Oct 29, 2020 at 1:04 PM Zi Yan <zi.yan@xxxxxxxx> wrote:
>>
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> In isolate_migratepages_block, when cc->alloc_contig is true, we are
>> able to isolate compound pages, nr_migratepages and nr_isolated did not
>> count compound pages correctly, causing us to isolate more pages than we
>> thought. Use thp_nr_pages to count pages. Otherwise, we might be trapped
>> in too_many_isolated while loop, since the actual isolated pages can go
>> up to COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,
>
> Is it that easy to run into? 16384 doesn't seem like too many pages, just 64MB.

I hit this when I was running oom01 from ltp to test my PUD THP patchset, which
allocates PUD THPs from CMA regions and splits them into PMD THPs due to memory
pressure. I am not sure if it is common that in the upstream kernel PMD THPs will
be allocated in CMA regions due to allocation fallback.

>
>> since we stop isolation after cc->nr_migratepages reaches to
>> COMPACT_CLUSTER_MAX.
>>
>> In addition, after we fix the issue above, cc->nr_migratepages could
>> never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
>> thus page isolation could not stop as we intended. Change the isolation
>> stop condition to >=.
>
> The fix looks sane to me. Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

Thanks.

>
> Shall you add Fixes tag to commit
> 1da2f328fa643bd72197dfed0c655148af31e4eb? And may cc stable.

Sure.

Fixes: 1da2f328fa64 (“mm,thp,compaction,cma: allow THP migration for CMA allocations”)

stable cc’ed.

>
>>
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>  mm/compaction.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index ee1f8439369e..0683a4999581 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>
>>  isolate_success:
>>                 list_add(&page->lru, &cc->migratepages);
>> -               cc->nr_migratepages++;
>> -               nr_isolated++;
>> +               cc->nr_migratepages += thp_nr_pages(page);
>> +               nr_isolated += thp_nr_pages(page);
>>
>>                 /*
>>                  * Avoid isolating too much unless this block is being
>> @@ -1021,7 +1021,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>                  * or a lock is contended. For contention, isolate quickly to
>>                  * potentially remove one source of contention.
>>                  */
>> -               if (cc->nr_migratepages == COMPACT_CLUSTER_MAX &&
>> +               if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX &&
>>                     !cc->rescan && !cc->contended) {
>>                         ++low_pfn;
>>                         break;
>> @@ -1132,7 +1132,7 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
>>                 if (!pfn)
>>                         break;
>>
>> -               if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
>> +               if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX)
>>                         break;
>>         }
>>
>> --
>> 2.28.0
>>
>>


—
Best Regards,
Yan Zi

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