writeback, cgroup: support switching multiple inodes at once

Currently only a single inode can be switched to another writeback
structure at once.  That means to switch an inode a separate
inode_switch_wbs_context structure must be allocated, and a separate rcu
callback and work must be scheduled.

It's fine for the existing ad-hoc switching, which is not happening that
often, but sub-optimal for massive switching required in order to release
a writeback structure.  To prepare for it, let's add a support for
switching multiple inodes at once.

Instead of containing a single inode pointer, inode_switch_wbs_context
will contain a NULL-terminated array of inode pointers.
inode_do_switch_wbs() will be called for each inode.

To optimize the locking bdi->wb_switch_rwsem, old_wb's and new_wb's
list_locks will be acquired and released only once altogether for all
inodes.  wb_wakeup() will be also be called only once.  Instead of calling
wb_put(old_wb) after each successful switch, wb_put_many() is introduced
and used.

Link: https://lkml.kernel.org/r/20210608230225.2078447-8-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
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, 16 insertions, 2 deletions

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index e5dc238ebe4f..63f52ad2ce7a 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -240,8 +240,9 @@ static inline void wb_get(struct bdi_writeback *wb)
 /**
  * wb_put - decrement a wb's refcount
  * @wb: bdi_writeback to put
+ * @nr: number of references to put
  */
-static inline void wb_put(struct bdi_writeback *wb)
+static inline void wb_put_many(struct bdi_writeback *wb, unsigned long nr)
 {
 	if (WARN_ON_ONCE(!wb->bdi)) {
 		/*
@@ -252,7 +253,16 @@ static inline void wb_put(struct bdi_writeback *wb)
 	}
 
 	if (wb != &wb->bdi->wb)
-		percpu_ref_put(&wb->refcnt);
+		percpu_ref_put_many(&wb->refcnt, nr);
+}
+
+/**
+ * wb_put - decrement a wb's refcount
+ * @wb: bdi_writeback to put
+ */
+static inline void wb_put(struct bdi_writeback *wb)
+{
+	wb_put_many(wb, 1);
 }
 
 /**
@@ -281,6 +291,10 @@ static inline void wb_put(struct bdi_writeback *wb)
 {
 }
 
+static inline void wb_put_many(struct bdi_writeback *wb, unsigned long nr)
+{
+}
+
 static inline bool wb_dying(struct bdi_writeback *wb)
 {
 	return false;