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

[Zi Yan <ziy@nvidia.com>]

On 22 Sep 2025, at 22:35, Andrew Morton wrote:

> There has been a lot of discussion here, little clarity and no recorded
> acks.  I'm inclined to just drop the patch and to ask that issue be
> reopened.
>
> Comments?
>
> Thanks.
>
>
> From: Wei Yang <richard.weiyang@gmail.com>
> Subject: mm/compaction: fix low_pfn advance on isolating hugetlb
> Date: Wed, 10 Sep 2025 09:22:40 +0000
>
> 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.
>
> Link: https://lkml.kernel.org/r/20250910092240.3981-1-richard.weiyang@gmail.com
> Fixes: 56ae0bb349b4 ("mm: compaction: convert to use a folio in isolate_migratepages_block()")
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Brendan Jackman <jackmanb@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Wei Yang <richard.weiyang@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
>  mm/compaction.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- a/mm/compaction.c~mm-compaction-fix-low_pfn-advance-on-isolating-hugetlb
> +++ a/mm/compaction.c
> @@ -989,7 +989,7 @@ isolate_migratepages_block(struct compac
>  				 * 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;
>  				goto isolate_success_no_list;
>  			}
>
> _

Acked-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi