Re: [PATCH 4/5] ceph: Assert writeback loop invariants

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

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 =