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 the
previous two patches which fix oopses caused by `pages` persisting into
the next loop iteration.

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 | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index 91cc43950162..b3569d44d510 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1669,7 +1669,9 @@ 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:
@@ -1703,11 +1705,10 @@ static int ceph_writepages_start(struct address_space *mapping,
 		}
 
 		rc = ceph_submit_write(mapping, wbc, &ceph_wbc);
-		if (rc)
-			goto release_folios;
-
 		ceph_wbc.locked_pages = 0;
 		ceph_wbc.strip_unit_end = 0;
+		if (rc)
+			goto release_folios;
 
 		if (folio_batch_count(&ceph_wbc.fbatch) > 0) {
 			ceph_wbc.nr_folios =
-- 
2.51.2

On Tue, 2025-12-30 at 18:43 -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 the
> previous two patches which fix oopses caused by `pages` persisting into
> the next loop iteration.
> 
> 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 | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 91cc43950162..b3569d44d510 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1669,7 +1669,9 @@ 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);
> +

It's not good idea to introduce BUG_ON() in write pages logic. I am definitely
against these two BUG_ON() here.

>  		ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
>  
>  get_more_pages:
> @@ -1703,11 +1705,10 @@ static int ceph_writepages_start(struct address_space *mapping,
>  		}
>  
>  		rc = ceph_submit_write(mapping, wbc, &ceph_wbc);
> -		if (rc)
> -			goto release_folios;
> -

Frankly speaking, its' completely not clear the from commit message why we move
this check. What's the problem is here? How this move can fix the issue?

Thanks,
Slava.

>  		ceph_wbc.locked_pages = 0;
>  		ceph_wbc.strip_unit_end = 0;
> +		if (rc)
> +			goto release_folios;
>  
>  		if (folio_batch_count(&ceph_wbc.fbatch) > 0) {
>  			ceph_wbc.nr_folios =

On Mon, Jan 5, 2026 at 2:29 PM Viacheslav Dubeyko <Slava.Dubeyko@ibm.com> wrote:
>
> On Tue, 2025-12-30 at 18:43 -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 the
> > previous two patches which fix oopses caused by `pages` persisting into
> > the next loop iteration.
> >
> > 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 | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> > index 91cc43950162..b3569d44d510 100644
> > --- a/fs/ceph/addr.c
> > +++ b/fs/ceph/addr.c
> > @@ -1669,7 +1669,9 @@ 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);
> > +
>

Hi Slava,

> It's not good idea to introduce BUG_ON() in write pages logic. I am definitely
> against these two BUG_ON() here.

I share your distaste for BUG_ON() in writeback. However, the
BUG_ON(ceph_wbc.pages); already exists in ceph_allocate_page_array().
This patch is trying to catch that earlier, where it's easier to
troubleshoot. If I changed these to WARN_ON(), it would not prevent
the oops.

And the writeback code has a lot of BUG_ON() checks already (so I am
not "introducing" BUG_ON), but I suppose you could be saying "it's
already a problem, please don't make it worse."

If I introduce a ceph_discard_page_array() function (as discussed on
patch 4), I could call it at the top of the loop (to *ensure* a clean
state) instead of using BUG_ON() (to *assert* a clean state). I'd like
to hear from other reviewers which approach they'd prefer.

>
> >               ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
> >
> >  get_more_pages:
> > @@ -1703,11 +1705,10 @@ static int ceph_writepages_start(struct address_space *mapping,
> >               }
> >
> >               rc = ceph_submit_write(mapping, wbc, &ceph_wbc);
> > -             if (rc)
> > -                     goto release_folios;
> > -
>
> Frankly speaking, its' completely not clear the from commit message why we move
> this check. What's the problem is here? How this move can fix the issue?

The diff makes it a little unclear, but I'm actually moving
ceph_wbc.{locked_pages,strip_unit_end} = 0; *above* the check (see
commit message: "also make it the previous iteration's responsibility
to guarantee [locked_pages is] reset") so that they happen
unconditionally. Git just happens to see it in terms of the if/goto
moving downward, not the assignments moving up.

Warm regards,
Sam


>
> Thanks,
> Slava.
>
> >               ceph_wbc.locked_pages = 0;
> >               ceph_wbc.strip_unit_end = 0;
> > +             if (rc)
> > +                     goto release_folios;
> >
> >               if (folio_batch_count(&ceph_wbc.fbatch) > 0) {
> >                       ceph_wbc.nr_folios =