[PATCH v2] ceph: fix leaked inode reference on writeback abort at umount

ceph_dirty_folio() takes a wrbuffer claim on each newly dirtied folio: it
bumps i_wrbuffer_ref (taking an ihold() on the 0->1 transition) and
attaches the snap_context to folio->private.  That claim is released only
by ceph_put_wrbuffer_cap_refs(), which for a submitted write runs from
writepages_finish().

In ceph_submit_write(), if ceph_inc_osd_stopping_blocker() fails -- which
happens during umount -- the request is aborted before submission: the
already-collected folios are only redirtied and unlocked, so
writepages_finish() never runs and the claim is leaked.
redirty_page_for_writepage() -> folio_redirty_for_writepage() ->
filemap_dirty_folio() sets PG_dirty directly and does not go through
->dirty_folio, so ceph_dirty_folio() is not re-entered to rebalance it.
Because every subsequent writeback also fails the osd_stopping_blocker,
i_wrbuffer_ref never returns to 0, the ihold() is never dropped, and the
inode cannot be evicted:

  VFS: Busy inodes after unmount of ceph
  kernel BUG at fs/super.c:650!

Release the orphaned claim in the abort path before redirtying, via
ceph_undo_wrbuffer_claim(): detach the snap_context, drop the wrbuffer
reference (letting i_wrbuffer_ref reach 0 and iput() the inode), and drop
the snap_context reference -- i.e. do what writepages_finish() would have
done for these never-submitted folios.

Only the locked_pages entries are undone; folios still in the fbatch were
never dirty-cleared by this call (folio_clear_dirty_for_io() is the
ownership-transfer point, and a successful move NULLs the fbatch slot), so
they hold no claim this call owns.

Fixes: fd7449d937e7 ("ceph: fix generic/421 test failure")
Signed-off-by: Matthew Brown <[email protected]>
---

v2:
 - Reword the commit message: the leak is an orphaned wrbuffer claim on an
   unsubmitted write (writepages_finish() never runs), not
   redirty_page_for_writepage() re-entering ceph_dirty_folio().  Thanks to
   Alex Markuze for the correction.
 - Convert ceph_undo_wrbuffer_claim() to the folio API
   (struct folio * / folio_detach_private()), per Alex Markuze.
 - Spell out the locked_pages/fbatch ownership split in the message, per
   Xiubo Li's comment.

v1: https://lore.kernel.org/all/[email protected]/
 fs/ceph/addr.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index ecf33b6..e9f561b 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1426,6 +1426,16 @@ void ceph_shift_unused_folios_left(struct folio_batch *fbatch)
 	fbatch->nr = n;
 }
 
+static void ceph_undo_wrbuffer_claim(struct inode *inode, struct folio *folio)
+{
+	struct ceph_snap_context *snapc = folio_detach_private(folio);
+
+	if (!snapc)
+		return;
+	ceph_put_wrbuffer_cap_refs(ceph_inode(inode), 1, snapc);
+	ceph_put_snap_context(snapc);
+}
+
 static
 int ceph_submit_write(struct address_space *mapping,
 			struct writeback_control *wbc,
@@ -1489,6 +1499,7 @@ int ceph_submit_write(struct address_space *mapping,
 			if (!page)
 				continue;
 
+			ceph_undo_wrbuffer_claim(inode, page_folio(page));
 			redirty_page_for_writepage(wbc, page);
 			unlock_page(page);
 		}
-- 
2.44.0