mm: simplify bdi refcounting

Move grabbing and releasing the bdi refcount out of the common
wb_init/wb_exit helpers into code that is only used for the non-default
memcg driven bdi_writeback structures.

[hch@lst.de: add comment]
Link: https://lkml.kernel.org/r/20211027074207.GA12793@lst.de
[akpm@linux-foundation.org: fix typo]

Link: https://lkml.kernel.org/r/20211021124441.668816-6-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 3320700..993c562 100644 (file)
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -103,6 +103,9 @@ struct wb_completion {
  * change as blkcg is disabled and enabled higher up in the hierarchy, a wb
  * is tested for blkcg after lookup and removed from index on mismatch so
  * that a new wb for the combination can be created.
+ *
+ * Each bdi_writeback that is not embedded into the backing_dev_info must hold
+ * a reference to the parent backing_dev_info.  See cgwb_create() for details.
  */
 struct bdi_writeback {
        struct backing_dev_info *bdi;   /* our parent bdi */

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 768e9ae..5ccb250 100644 (file)
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -291,8 +291,6 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
 
        memset(wb, 0, sizeof(*wb));
 
-       if (wb != &bdi->wb)
-               bdi_get(bdi);
        wb->bdi = bdi;
        wb->last_old_flush = jiffies;
        INIT_LIST_HEAD(&wb->b_dirty);
@@ -316,7 +314,7 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
 
        err = fprop_local_init_percpu(&wb->completions, gfp);
        if (err)
-               goto out_put_bdi;
+               return err;
 
        for (i = 0; i < NR_WB_STAT_ITEMS; i++) {
                err = percpu_counter_init(&wb->stat[i], 0, gfp);
@@ -330,9 +328,6 @@ out_destroy_stat:
        while (i--)
                percpu_counter_destroy(&wb->stat[i]);
        fprop_local_destroy_percpu(&wb->completions);
-out_put_bdi:
-       if (wb != &bdi->wb)
-               bdi_put(bdi);
        return err;
 }
 
@@ -373,8 +368,6 @@ static void wb_exit(struct bdi_writeback *wb)
                percpu_counter_destroy(&wb->stat[i]);
 
        fprop_local_destroy_percpu(&wb->completions);
-       if (wb != &wb->bdi->wb)
-               bdi_put(wb->bdi);
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK
@@ -397,6 +390,7 @@ static void cgwb_release_workfn(struct work_struct *work)
        struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
                                                release_work);
        struct blkcg *blkcg = css_to_blkcg(wb->blkcg_css);
+       struct backing_dev_info *bdi = wb->bdi;
 
        mutex_lock(&wb->bdi->cgwb_release_mutex);
        wb_shutdown(wb);
@@ -416,6 +410,7 @@ static void cgwb_release_workfn(struct work_struct *work)
 
        percpu_ref_exit(&wb->refcnt);
        wb_exit(wb);
+       bdi_put(bdi);
        WARN_ON_ONCE(!list_empty(&wb->b_attached));
        kfree_rcu(wb, rcu);
 }
@@ -497,6 +492,7 @@ static int cgwb_create(struct backing_dev_info *bdi,
        INIT_LIST_HEAD(&wb->b_attached);
        INIT_WORK(&wb->release_work, cgwb_release_workfn);
        set_bit(WB_registered, &wb->state);
+       bdi_get(bdi);
 
        /*
         * The root wb determines the registered state of the whole bdi and
@@ -528,6 +524,7 @@ static int cgwb_create(struct backing_dev_info *bdi,
        goto out_put;
 
 err_fprop_exit:
+       bdi_put(bdi);
        fprop_local_destroy_percpu(&wb->memcg_completions);
 err_ref_exit:
        percpu_ref_exit(&wb->refcnt);