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

[Zi Yan <ziy@nvidia.com>]

On 11 Sep 2025, at 20:28, Wei Yang wrote:

> On Thu, Sep 11, 2025 at 12:28:24PM -0400, Zi Yan wrote:
>> On 11 Sep 2025, at 2:30, Wei Yang wrote:
>>
>>> 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.
>>
>> OK, that sounds inefficient and inconsistent with your fix.
>>
>> While at it, can you also change two “low_pfn += (1UL << order) - 1” to skip
>> the rest of hugetlb?
>>
>
> Sure, glad to.
>
> You prefer do the fix in one patch or have a separate one?

A separate one is better, since these are optimizations, whereas your current
patch is a fix.


--
Best Regards,
Yan, Zi