The function move_dirty_folio_in_page_array() was created by commit
ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method") by
moving code from ceph_writepages_start() to this function.

This new function is supposed to return an error code which is checked
by the caller (now ceph_process_folio_batch()), and on error, the
caller invokes redirty_page_for_writepage() and then breaks from the
loop.

However, the refactoring commit has gone wrong, and it by accident, it
always returns 0 (= success) because it first NULLs the pointer and
then returns PTR_ERR(NULL) which is always 0.  This means errors are
silently ignored, leaving NULL entries in the page array, which may
later crash the kernel.

The simple solution is to call PTR_ERR() before clearing the pointer.

Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/ceph/addr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index 8b202d789e93..e3e0d477f3f7 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1264,7 +1264,9 @@ static inline int move_dirty_folio_in_page_array(struct address_space *mapping,
 								0,
 								gfp_flags);
 		if (IS_ERR(pages[index])) {
-			if (PTR_ERR(pages[index]) == -EINVAL) {
+			int err = PTR_ERR(pages[index]);
+
+			if (err == -EINVAL) {
 				pr_err_client(cl, "inode->i_blkbits=%hhu\n",
 						inode->i_blkbits);
 			}
@@ -1273,7 +1275,7 @@ static inline int move_dirty_folio_in_page_array(struct address_space *mapping,
 			BUG_ON(ceph_wbc->locked_pages == 0);
 
 			pages[index] = NULL;
-			return PTR_ERR(pages[index]);
+			return err;
 		}
 	} else {
 		pages[index] = &folio->page;
-- 
2.47.2

On Thu, Aug 28, 2025 at 1:16 PM Max Kellermann <max.kellermann@ionos.com> wrote:
>
> The function move_dirty_folio_in_page_array() was created by commit
> ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method") by
> moving code from ceph_writepages_start() to this function.
>
> This new function is supposed to return an error code which is checked
> by the caller (now ceph_process_folio_batch()), and on error, the
> caller invokes redirty_page_for_writepage() and then breaks from the
> loop.
>
> However, the refactoring commit has gone wrong, and it by accident, it
> always returns 0 (= success) because it first NULLs the pointer and
> then returns PTR_ERR(NULL) which is always 0.  This means errors are
> silently ignored, leaving NULL entries in the page array, which may
> later crash the kernel.
>
> The simple solution is to call PTR_ERR() before clearing the pointer.
>
> Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
> Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
>  fs/ceph/addr.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 8b202d789e93..e3e0d477f3f7 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1264,7 +1264,9 @@ static inline int move_dirty_folio_in_page_array(struct address_space *mapping,
>                                                                 0,
>                                                                 gfp_flags);
>                 if (IS_ERR(pages[index])) {
> -                       if (PTR_ERR(pages[index]) == -EINVAL) {
> +                       int err = PTR_ERR(pages[index]);
> +
> +                       if (err == -EINVAL) {
>                                 pr_err_client(cl, "inode->i_blkbits=%hhu\n",
>                                                 inode->i_blkbits);
>                         }
> @@ -1273,7 +1275,7 @@ static inline int move_dirty_folio_in_page_array(struct address_space *mapping,
>                         BUG_ON(ceph_wbc->locked_pages == 0);
>
>                         pages[index] = NULL;
> -                       return PTR_ERR(pages[index]);
> +                       return err;
>                 }
>         } else {
>                 pages[index] = &folio->page;
> --
> 2.47.2
>

Queued up for 6.17-rc6.

Thanks,

                Ilya