[PATCH v3 4/4] ceph: assert writeback loop invariants

[Sam Edwards <cfsworks@gmail.com>]

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.

This patch does not change invariants, behavior, or failure modes.
The added BUG_ON() lines catch conditions that would already trigger oops,
but do so earlier for easier debugging and programmer clarity.

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 cdf11288d6b7..4e392fc70d33 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1663,7 +1663,9 @@ 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:
-- 
2.52.0