writeback, cgroup: keep list of inodes attached to bdi_writeback

Currently there is no way to iterate over inodes attached to a specific
cgwb structure.  It limits the ability to efficiently reclaim the
writeback structure itself and associated memory and block cgroup
structures without scanning all inodes belonging to a sb, which can be
prohibitively expensive.

While dirty/in-active-writeback an inode belongs to one of the
bdi_writeback's io lists: b_dirty, b_io, b_more_io and b_dirty_time.  Once
cleaned up, it's removed from all io lists.  So the inode->i_io_list can
be reused to maintain the list of inodes, attached to a bdi_writeback
structure.

This patch introduces a new wb->b_attached list, which contains all inodes
which were dirty at least once and are attached to the given cgwb.  Inodes
attached to the root bdi_writeback structures are never placed on such
list.  The following patch will use this list to try to release cgwbs
structures more efficiently.

Link: https://lkml.kernel.org/r/20210608230225.2078447-6-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 576220acd686..54c5dc4b8c24 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -396,6 +396,7 @@ static void cgwb_release_workfn(struct work_struct *work)
 	fprop_local_destroy_percpu(&wb->memcg_completions);
 	percpu_ref_exit(&wb->refcnt);
 	wb_exit(wb);
+	WARN_ON_ONCE(!list_empty(&wb->b_attached));
 	kfree_rcu(wb, rcu);
 }
 
@@ -472,6 +473,7 @@ static int cgwb_create(struct backing_dev_info *bdi,
 
 	wb->memcg_css = memcg_css;
 	wb->blkcg_css = blkcg_css;
+	INIT_LIST_HEAD(&wb->b_attached);
 	INIT_WORK(&wb->release_work, cgwb_release_workfn);
 	set_bit(WB_registered, &wb->state);