[PATCH v6 1/3] writeback: clear the old wb's dirty IO state after switching inodes

[Liz Fong-Jones]

Switching a dirty inode to another wb can leave WB_has_dirty_io set on
the old one. inode_do_switch_wbs() moves the inode onto new_wb->b_dirty
through inode_io_list_move_locked(), which only updates new_wb, and
nothing calls wb_io_lists_depopulated() on old_wb. A live wb clears the
bit on its next dirty to clean transition, but a wb that gets no more
inodes, such as a dying one, is freed with its avg_write_bandwidth still
in bdi->tot_write_bandwidth, which wb_split_bdi_pages() and
wb_min_max_ratio() divide by.

Call wb_io_lists_depopulated(old_wb) after the switch loop in
process_inode_switch_wbs(), while old_wb->list_lock is still held.

Test: after the last patch in this series switched a replaced wb's dirty
inodes away, BdiWriteBandwidth stayed at 411700 and 407820 kBps with no
writers and b_dirty, b_io and b_more_io all empty; with this patch it
read 0, in 3 of 3 runs.

Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-5-5 checkpatch sparse
Assisted-by: Claude:claude-fable-5-1
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Liz Fong-Jones <lizf@xxxxxxxxxxxx>
---
 fs/fs-writeback.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d26b2cf05283..050d507aa554 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -566,6 +566,12 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
 		}
 	}
 
+	/*
+	 * inode_do_switch_wbs() only updates @new_wb's dirty IO state.  Clear
+	 * @old_wb's if its IO lists are now empty.
+	 */
+	wb_io_lists_depopulated(old_wb);
+
 	spin_unlock(&new_wb->list_lock);
 	spin_unlock(&old_wb->list_lock);
 

-- 
2.53.0