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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/9/12 09:13, Zi Yan wrote:
> 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.

I'm afraid we should not do that. Because the order getting from 
compound_order(page) is not stable without lock protection for a 
hugetlb, This is why we add a sanity check 'if (order <= 
MAX_PAGE_ORDER)' before advancing low_pfn.