If `locked_pages` is zero, the page array must not be allocated:
ceph_process_folio_batch() uses `locked_pages` to decide when to
allocate `pages`, and redundant allocations trigger
ceph_allocate_page_array()'s BUG_ON(), resulting in a worker oops (and
writeback stall) or even a kernel panic. Consequently, the main loop in
ceph_writepages_start() assumes that the lifetime of `pages` is confined
to a single iteration.

This expectation is currently not clear enough, as evidenced by two
recent patches which fix oopses caused by `pages` persisting into
the next loop iteration:
- "ceph: Do not propagate page array emplacement errors as batch errors"
- "ceph: Free page array when ceph_submit_write fails"

Use an explicit BUG_ON() at the top of the loop to assert the loop's
preexisting expectation that `pages` is cleaned up by the previous
iteration. Because this is closely tied to `locked_pages`, also make it
the previous iteration's responsibility to guarantee its reset, and
verify with a second new BUG_ON() instead of handling (and masking)
failures to do so.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
---
 fs/ceph/addr.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index 3becb13a09fe..f2db05b51a3b 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1679,7 +1679,9 @@ static int ceph_writepages_start(struct address_space *mapping,
 		tag_pages_for_writeback(mapping, ceph_wbc.index, ceph_wbc.end);
 
 	while (!has_writeback_done(&ceph_wbc)) {
-		ceph_wbc.locked_pages = 0;
+		BUG_ON(ceph_wbc.locked_pages);
+		BUG_ON(ceph_wbc.pages);
+
 		ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
 
 get_more_pages:
-- 
2.51.2

On Wed, 2026-01-07 at 13:01 -0800, Sam Edwards wrote:
> If `locked_pages` is zero, the page array must not be allocated:
> ceph_process_folio_batch() uses `locked_pages` to decide when to
> allocate `pages`, and redundant allocations trigger
> ceph_allocate_page_array()'s BUG_ON(), resulting in a worker oops (and
> writeback stall) or even a kernel panic. Consequently, the main loop in
> ceph_writepages_start() assumes that the lifetime of `pages` is confined
> to a single iteration.
> 
> This expectation is currently not clear enough, as evidenced by two
> recent patches which fix oopses caused by `pages` persisting into
> the next loop iteration:
> - "ceph: Do not propagate page array emplacement errors as batch errors"
> - "ceph: Free page array when ceph_submit_write fails"
> 
> Use an explicit BUG_ON() at the top of the loop to assert the loop's
> preexisting expectation that `pages` is cleaned up by the previous
> iteration. Because this is closely tied to `locked_pages`, also make it
> the previous iteration's responsibility to guarantee its reset, and
> verify with a second new BUG_ON() instead of handling (and masking)
> failures to do so.
> 
> Signed-off-by: Sam Edwards <CFSworks@gmail.com>
> ---
>  fs/ceph/addr.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 3becb13a09fe..f2db05b51a3b 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1679,7 +1679,9 @@ static int ceph_writepages_start(struct address_space *mapping,
>  		tag_pages_for_writeback(mapping, ceph_wbc.index, ceph_wbc.end);
>  
>  	while (!has_writeback_done(&ceph_wbc)) {
> -		ceph_wbc.locked_pages = 0;
> +		BUG_ON(ceph_wbc.locked_pages);
> +		BUG_ON(ceph_wbc.pages);
> +
>  		ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
>  
>  get_more_pages:

I don't agree with using BUG_ON() here.

Thanks,
Slava.