From 90c96612460e6accddbda5a195e6ddaa391ecde0 Mon Sep 17 00:00:00 2001 From: Liz Fong-Jones Date: Sat, 17 Oct 2026 04:07:11 +0000 Subject: [PATCH] writeback: keep cleanup_offline_cgwb()'s scan out of syncs On a filesystem mounted with lazytime, sync() and syncfs() can return without persisting timestamp updates of inodes attached to a dying cgroup wb, so a crash after the sync loses them. File data and sizes are not affected. Since commit f6988c90671e ("writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes"), isw_prepare_wbs_switch() moves the inodes it walks to the newest end of the list. When a pass fills its isw before reaching an inode that got its lazytime update after a sync's cutoff, the walked inodes land past it on b_dirty_time, and the sync's queue_io(), which stops at the first inode dirtied after the cutoff, misses them. The late inode has to be one without a pending lazytime update; one already on b_dirty_time keeps its position. The window is narrow. cleanup_offline_cgwbs_workfn() skips wbs with anything on b_dirty, b_io or b_more_io, so the scan has to land between wb_writeback() fixing dirtied_before and taking list_lock for its first queue_io(), unless a multi-pass drain started before the wb got dirty IO. Take wb_switch_rwsem for read across the scan, as process_inode_switch_wbs() does for the switches. Fixes: f6988c90671e ("writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes") Cc: stable@vger.kernel.org Suggested-by: Tejun Heo Link: https://lore.kernel.org/all/0bc5d58533f10b4841807be3025daa11@kernel.org/ Acked-by: Tejun Heo Signed-off-by: Liz Fong-Jones Link: https://patch.msgid.link/20261017040710.1174009-1-lizf@honeycomb.io Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) --- fs/fs-writeback.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 1f192a581fb2..ac612c02ced9 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -790,6 +790,13 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */ nr = 0; + /* + * The scan moves walked inodes to the newest end of b_dirty_time, + * past ones it didn't reach. If one of those was dirtied after a + * sync's cutoff, the sync's queue_io() stops there and misses the + * moved ones. Exclude syncs as process_inode_switch_wbs() does. + */ + down_read(&wb->bdi->wb_switch_rwsem); spin_lock(&wb->list_lock); /* * In addition to the inodes that have completed writeback, also switch @@ -804,6 +811,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) restart = isw_prepare_wbs_switch(new_wb, isw, &wb->b_dirty_time, &nr); spin_unlock(&wb->list_lock); + up_read(&wb->bdi->wb_switch_rwsem); /* no attached inodes? bail out */ if (nr == 0) {