* Re: [PATCH v3] mm/migrate: fix shmem xarray update during migration
2025-03-05 20:04 [PATCH v3] mm/migrate: fix shmem xarray update during migration Zi Yan
@ 2025-03-05 20:31 ` Matthew Wilcox
2025-03-08 3:03 ` Baolin Wang
2025-03-08 3:17 ` Liu Shixin
2 siblings, 0 replies; 4+ messages in thread
From: Matthew Wilcox @ 2025-03-05 20:31 UTC (permalink / raw)
To: Zi Yan
Cc: Liu Shixin, Baolin Wang, linux-mm, Andrew Morton, Barry Song,
David Hildenbrand, Kefeng Wang, Lance Yang, Ryan Roberts,
Hugh Dickins, Charan Teja Kalla, linux-kernel, stable
On Wed, Mar 05, 2025 at 03:04:03PM -0500, 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@casper.infradead.org/
>
> 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@huawei.com>
> Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Cc: stable@vger.kernel.org
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH v3] mm/migrate: fix shmem xarray update during migration
2025-03-05 20:04 [PATCH v3] mm/migrate: fix shmem xarray update during migration Zi Yan
2025-03-05 20:31 ` Matthew Wilcox
@ 2025-03-08 3:03 ` Baolin Wang
2025-03-08 3:17 ` Liu Shixin
2 siblings, 0 replies; 4+ messages in thread
From: Baolin Wang @ 2025-03-08 3:03 UTC (permalink / raw)
To: Zi Yan, Liu Shixin, linux-mm
Cc: Andrew Morton, Barry Song, David Hildenbrand, Kefeng Wang,
Lance Yang, Ryan Roberts, Matthew Wilcox, Hugh Dickins,
Charan Teja Kalla, linux-kernel, stable
On 2025/3/6 04:04, 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@casper.infradead.org/
>
> 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@huawei.com>
> Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Cc: stable@vger.kernel.org
Thanks for fixing the issue.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> 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;
> }
>
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH v3] mm/migrate: fix shmem xarray update during migration
2025-03-05 20:04 [PATCH v3] mm/migrate: fix shmem xarray update during migration Zi Yan
2025-03-05 20:31 ` Matthew Wilcox
2025-03-08 3:03 ` Baolin Wang
@ 2025-03-08 3:17 ` Liu Shixin
2 siblings, 0 replies; 4+ messages in thread
From: Liu Shixin @ 2025-03-08 3:17 UTC (permalink / raw)
To: Zi Yan, Baolin Wang, linux-mm
Cc: Andrew Morton, Barry Song, David Hildenbrand, Kefeng Wang,
Lance Yang, Ryan Roberts, Matthew Wilcox, Hugh Dickins,
Charan Teja Kalla, linux-kernel, stable
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@casper.infradead.org/
>
> 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@huawei.com>
> Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Cc: stable@vger.kernel.org
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;
> }
>
^ permalink raw reply [flat|nested] 4+ messages in thread