Re: [PATCH] mm/compaction: fix low_pfn advance on isolating hugetlb

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Sep 11, 2025 at 03:34:13AM +0000, Wei Yang wrote:
>On Wed, Sep 10, 2025 at 09:50:09PM -0400, Zi Yan wrote:
>>On 10 Sep 2025, at 21:38, Zi Yan wrote:
>>
>>> On 10 Sep 2025, at 21:35, Zi Yan wrote:
>>>
>>>> On 10 Sep 2025, at 21:25, Wei Yang wrote:
>>>>
>>>>> On Wed, Sep 10, 2025 at 09:22:40AM +0000, Wei Yang wrote:
>>>>>> Commit 56ae0bb349b4 ("mm: compaction: convert to use a folio in
>>>>>> isolate_migratepages_block()") converts api from page to folio. But the
>>>>>> low_pfn advance for hugetlb page seems wrong when low_pfn doesn't point
>>>>>> to head page.
>>>>>>
>>>>>> Originally, if page is a hugetlb tail page, compound_nr() return 1,
>>>>>> which means low_pfn only advance one in next iteration. After the
>>>>>> change, low_pfn would advance more than the hugetlb range, since
>>>>>> folio_nr_pages() always return total number of the large page. This
>>>>>> results in skipping some range to isolate and then to migrate.
>>>>>>
>>>>>> The worst case for alloc_contig is it does all the isolation and
>>>>>> migration, but finally find some range is still not isolated. And then
>>>>>> undo all the work and try a new range.
>>>>>>
>>>>>> Advance low_pfn to the end of hugetlb.
>>>>>>
>>>>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>>>>> Fixes: 56ae0bb349b4 ("mm: compaction: convert to use a folio in isolate_migratepages_block()")
>>
>>This behavior seems to be introduced by commit 369fa227c219 ("mm: make
>>alloc_contig_range handle free hugetlb pages”). The related change is:
>>
>>+               if (PageHuge(page) && cc->alloc_contig) {
>>+                       ret = isolate_or_dissolve_huge_page(page);
>>+
>>+                       /*
>>+                        * Fail isolation in case isolate_or_dissolve_huge_page()
>>+                        * reports an error. In case of -ENOMEM, abort right away.
>>+                        */
>>+                       if (ret < 0) {
>>+                                /* Do not report -EBUSY down the chain */
>>+                               if (ret == -EBUSY)
>>+                                       ret = 0;
>>+                               low_pfn += (1UL << compound_order(page)) - 1;
>
>The compound_order(page) return 1 for a tail page.
>
>See below.
>
>>+                               goto isolate_fail;
>>+                       }
>>+
>>+                       /*
>>+                        * Ok, the hugepage was dissolved. Now these pages are
>>+                        * Buddy and cannot be re-allocated because they are
>>+                        * isolated. Fall-through as the check below handles
>>+                        * Buddy pages.
>>+                        */
>>+               }
>>+
>>
>>>>>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>>> Cc: Oscar Salvador <osalvador@suse.de>
>>>>>
>>>>> Forgot to cc stable.
>>>>>
>>>>> Cc: <stable@vger.kernel.org>
>>>>
>>>> Is there any bug report to justify the backport? Since it is more likely
>>>> to be a performance issue instead of a correctness issue.
>>>>
>>>>>
>>>>>> ---
>>>>>> mm/compaction.c | 2 +-
>>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>>>>> index bf021b31c7ec..1e8f8eca318c 100644
>>>>>> --- a/mm/compaction.c
>>>>>> +++ b/mm/compaction.c
>>>>>> @@ -989,7 +989,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>>>>> 				 * Hugepage was successfully isolated and placed
>>>>>> 				 * on the cc->migratepages list.
>>>>>> 				 */
>>>>>> -				low_pfn += folio_nr_pages(folio) - 1;
>>>>>> +				low_pfn += folio_nr_pages(folio) - folio_page_idx(folio, page) - 1;
>>>>>
>>>>> One question is why we advance compound_nr() in original version.
>>>>>
>>>>> Yes, there are several places advancing compound_nr(), but it seems to iterate
>>>>> on the same large page and do the same thing and advance 1 again.
>>>>>
>>>>> Not sure which part story I missed.
>>>>
>>>> isolate_migratepages_block() starts from the beginning of a pageblock.
>>>> How likely the code hit in the middle of a hugetlb?
>>>>
>>>
>>> In addition, there are two other “low_pfn += (1UL << order) - 1”
>>> in the if (PageHuge(page)), why not change them too if you think
>>> page can point to the middle of a hugetlb?
>>>
>
>The order here is get from compound_order(page), which is 1 for a tail page.
>
>So it looks right. Maybe I misunderstand it?

Oops, compound_order(page) returns 0 for tail page.

What I want to say is low_pfn advance by 1 for tail page. Sorry for the
misleading.

-- 
Wei Yang
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