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

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

On Thu, Sep 11, 2025 at 12:19:34PM -0400, Zi Yan wrote:
> On 10 Sep 2025, at 23:27, Wei Yang wrote:
> 
> > On Wed, Sep 10, 2025 at 09:35:53PM -0400, 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()")
> >>>> 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.
> >>
> >
> > OK, I thought cc-stable is paired with fixes tag.
> >
> > If not, please drop it.
> >
> >>>
> >>>> ---
> >>>> 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?
> >>
> >
> > OK, this is a kind of optimization based on the knowledge it is not likely to
> > be a tail page?
> 
> No, it might be that most of the time page is the head, or people assume so.

For compound pages, we will always have tail pfn < head pfn, so we should
always find the head page first.

If you did find a case where we somehow encounter a tail page here, I'd
love to see it. And then you'd also want to make sure the other compaction
trackers are appropriately accounted for.