[patch 046/262] mm: simplify bdi refcounting

Andrew Morton
2021-11-05T20:37:08Z

From: Christoph Hellwig <hch@lst.de>
Subject: 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>
---

include/linux/backing-dev-defs.h | 3 +++
mm/backing-dev.c | 13 +++++--------
2 files changed, 8 insertions(+), 8 deletions(-)

--- a/include/linux/backing-dev-defs.h~mm-simplify-bdi-refcounting
+++ a/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 */
--- a/mm/backing-dev.c~mm-simplify-bdi-refcounting
+++ a/mm/backing-dev.c
@@ -291,8 +291,6 @@ static int wb_init(struct bdi_writeback

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

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
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 w
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 w

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_de
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_de
goto out_put;

err_fprop_exit:
+ bdi_put(bdi);
fprop_local_destroy_percpu(&wb->memcg_completions);
err_ref_exit:
percpu_ref_exit(&wb->refcnt);
_