Re: [PATCH v2] mm/migrate: fix shmem xarray update during migration

[Zi Yan]

On 28 Feb 2025, at 12:49, Zi Yan wrote:

> Pagecache uses multi-index entries for large folio, so does shmem. Only
> swap cache still stores multiple entries for a single large folio.
> Commit fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
> fixed swap cache but got shmem wrong by storing multiple entries for
> a large shmem folio. Fix it by storing a single entry for a shmem
> folio.
>
> Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
> Reported-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@xxxxxxxxxx/
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Shivank Garg <shivankg@xxxxxxx>

+Cc:stable

> ---
>  mm/migrate.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 365c6daa8d1b..2c9669135a38 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -524,7 +524,11 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>  			folio_set_swapcache(newfolio);
>  			newfolio->private = folio_get_private(folio);
>  		}
> -		entries = nr;
> +		/* shmem uses high-order entry */
> +		if (!folio_test_anon(folio))
> +			entries = 1;
> +		else
> +			entries = nr;
>  	} else {
>  		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
>  		entries = 1;
> -- 
> 2.47.2


Best Regards,
Yan, Zi