Re: [PATCH v3] mm/migrate: fix shmem xarray update during migration

[Liu Shixin]

On 2025/3/6 4:04, Zi Yan wrote:
> A shmem folio can be either in page cache or in swap cache, but not at the
> same time. Namely, once it is in swap cache, folio->mapping should be NULL,
> and the folio is no longer in a shmem mapping.
>
> In __folio_migrate_mapping(), to determine the number of xarray entries
> to update, folio_test_swapbacked() is used, but that conflates shmem in
> page cache case and shmem in swap cache case. It leads to xarray
> multi-index entry corruption, since it turns a sibling entry to a
> normal entry during xas_store() (see [1] for a userspace reproduction).
> Fix it by only using folio_test_swapcache() to determine whether xarray
> is storing swap cache entries or not to choose the right number of xarray
> entries to update.
>
> [1] https://lore.kernel.org/linux-mm/Z8idPCkaJW1IChjT@xxxxxxxxxxxxxxxxxxxx/
>
> Note:
> In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is used
> to get swap_cache address space, but that ignores the shmem folio in swap
> cache case. It could lead to NULL pointer dereferencing when a
> in-swap-cache shmem folio is split at __xa_store(), since
> !folio_test_anon() is true and folio->mapping is NULL. But fortunately,
> its caller split_huge_page_to_list_to_order() bails out early with EBUSY
> when folio->mapping is NULL. So no need to take care of it here.
>
> 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/
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
Thanks for the patch, it works for me.
> ---
>  mm/migrate.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index fb4afd31baf0..c0adea67cd62 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -518,15 +518,13 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>  	if (folio_test_anon(folio) && folio_test_large(folio))
>  		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
>  	folio_ref_add(newfolio, nr); /* add cache reference */
> -	if (folio_test_swapbacked(folio)) {
> +	if (folio_test_swapbacked(folio))
>  		__folio_set_swapbacked(newfolio);
> -		if (folio_test_swapcache(folio)) {
> -			folio_set_swapcache(newfolio);
> -			newfolio->private = folio_get_private(folio);
> -		}
> +	if (folio_test_swapcache(folio)) {
> +		folio_set_swapcache(newfolio);
> +		newfolio->private = folio_get_private(folio);
>  		entries = nr;
>  	} else {
> -		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
>  		entries = 1;
>  	}
>