Re: [PATCH 2/2] mm/migrate: exclude hugetlb folios from MTHP_STAT_NR_ANON accounting

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

On 7/3/26 1:25 AM, Nico Pache wrote:
> __folio_migrate_mapping() increments MTHP_STAT_NR_ANON for the
> destination folio when `folio_test_anon(folio) && folio_test_large(folio)`
> is true. However, hugetlb folios satisfy both conditions despite having a
> completely separate accounting system; they use hugetlb_add_anon_rmap()
> which does not touch mTHP stats, and their free path also bypasses the
> mTHP decrement in __free_pages_prepare().
> 
> This causes MTHP_STAT_NR_ANON to be incremented on each hugetlb
> migration without a corresponding decrement, permanently inflating the
> nr_anon counter.
> 
> Add a !folio_test_hugetlb() check to both places in
> __folio_migrate_mapping() so that only actual mTHP folios are counted.
> 
> Fixes: 5d65c8d758f2 ("mm: count the number of anonymous THPs per size")
> Co-developed-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>   mm/migrate.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d9b23909d716..9fd50ea25d2d 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -590,7 +590,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>   		/* No turning back from here */
>   		newfolio->index = folio->index;
>   		newfolio->mapping = folio->mapping;
> -		if (folio_test_anon(folio) && folio_test_large(folio))
> +		if (folio_test_anon(folio) && folio_test_large(folio) &&
> +		    !folio_test_hugetlb(folio))
>   			mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);

Yes, anonymous hugetlb mappings can reach here, so this check looks 
reasonable to me.

>   		if (folio_test_swapbacked(folio))
>   			__folio_set_swapbacked(newfolio);
> @@ -623,7 +624,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>   	 */
>   	newfolio->index = folio->index;
>   	newfolio->mapping = folio->mapping;
> -	if (folio_test_anon(folio) && folio_test_large(folio))
> +	if (folio_test_anon(folio) && folio_test_large(folio) &&
> +	    !folio_test_hugetlb(folio))

If the hugetlb folio has a non-NULL mapping (i.e., it's a shared 
mapping), we should migrate it via 
hugetlbfs_migrate_folio()->migrate_huge_page_move_mapping(). In other 
words, shared hugetlb mappings should not reach this path, so this 
hugetlb check can be dropped. Or am I missing something?

>   		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
>   	folio_ref_add(newfolio, nr); /* add cache reference */
>   	if (folio_test_swapbacked(folio))