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

[Zi Yan <ziy@nvidia.com>]

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;
+                               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?
>
> Best Regards,
> Yan, Zi


Best Regards,
Yan, Zi