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

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

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>

>---
> 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.

> 				goto isolate_success_no_list;
> 			}
> 
>-- 
>2.34.1

-- 
Wei Yang
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