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

[Matthew Wilcox]

On Fri, Feb 28, 2025 at 10:42:19AM -0500, Zi Yan wrote:
> @@ -524,7 +525,11 @@ 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 (shmem_mapping(mapping))

It's be cheaper to check folio_test_anon() here, right?

Also, how did this bug remain unnoticed for almost 4 years?
Our testing is terrible ;-(

> +			entries = 1;
> +		else
> +			entries = nr;
>  	} else {
>  		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
>  		entries = 1;
> -- 
> 2.47.2
>