Re: [PATCH v2 5/6] ceph: Assert writeback loop invariants

Viacheslav Dubeyko
2026-01-08T20:12:38Z

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.