[PATCH 3/5] writeback: Separate out wb_get_lookup() from wb_get_create()

From: Tejun Heo
Date: Thu Aug 15 2019 - 15:58:27 EST


Separate out wb_get_lookup() which doesn't try to create one if there
isn't already one from wb_get_create(). This will be used by later
patches.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
include/linux/backing-dev.h | 2 +
mm/backing-dev.c | 55 +++++++++++++++++++++++++++++---------------
2 files changed, 39 insertions(+), 18 deletions(-)

--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -230,6 +230,8 @@ static inline int bdi_sched_wait(void *w
struct bdi_writeback_congested *
wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp);
void wb_congested_put(struct bdi_writeback_congested *congested);
+struct bdi_writeback *wb_get_lookup(struct backing_dev_info *bdi,
+ struct cgroup_subsys_state *memcg_css);
struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
struct cgroup_subsys_state *memcg_css,
gfp_t gfp);
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -618,13 +618,12 @@ out_put:
}

/**
- * wb_get_create - get wb for a given memcg, create if necessary
+ * wb_get_lookup - get wb for a given memcg
* @bdi: target bdi
* @memcg_css: cgroup_subsys_state of the target memcg (must have positive ref)
- * @gfp: allocation mask to use
*
- * Try to get the wb for @memcg_css on @bdi. If it doesn't exist, try to
- * create one. The returned wb has its refcount incremented.
+ * Try to get the wb for @memcg_css on @bdi. The returned wb has its
+ * refcount incremented.
*
* This function uses css_get() on @memcg_css and thus expects its refcnt
* to be positive on invocation. IOW, rcu_read_lock() protection on
@@ -641,6 +640,39 @@ out_put:
* each lookup. On mismatch, the existing wb is discarded and a new one is
* created.
*/
+struct bdi_writeback *wb_get_lookup(struct backing_dev_info *bdi,
+ struct cgroup_subsys_state *memcg_css)
+{
+ struct bdi_writeback *wb;
+
+ if (!memcg_css->parent)
+ return &bdi->wb;
+
+ rcu_read_lock();
+ wb = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
+ if (wb) {
+ struct cgroup_subsys_state *blkcg_css;
+
+ /* see whether the blkcg association has changed */
+ blkcg_css = cgroup_get_e_css(memcg_css->cgroup, &io_cgrp_subsys);
+ if (unlikely(wb->blkcg_css != blkcg_css || !wb_tryget(wb)))
+ wb = NULL;
+ css_put(blkcg_css);
+ }
+ rcu_read_unlock();
+
+ return wb;
+}
+
+/**
+ * wb_get_create - get wb for a given memcg, create if necessary
+ * @bdi: target bdi
+ * @memcg_css: cgroup_subsys_state of the target memcg (must have positive ref)
+ * @gfp: allocation mask to use
+ *
+ * Try to get the wb for @memcg_css on @bdi. If it doesn't exist, try to
+ * create one. See wb_get_lookup() for more details.
+ */
struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
struct cgroup_subsys_state *memcg_css,
gfp_t gfp)
@@ -653,20 +685,7 @@ struct bdi_writeback *wb_get_create(stru
return &bdi->wb;

do {
- rcu_read_lock();
- wb = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
- if (wb) {
- struct cgroup_subsys_state *blkcg_css;
-
- /* see whether the blkcg association has changed */
- blkcg_css = cgroup_get_e_css(memcg_css->cgroup,
- &io_cgrp_subsys);
- if (unlikely(wb->blkcg_css != blkcg_css ||
- !wb_tryget(wb)))
- wb = NULL;
- css_put(blkcg_css);
- }
- rcu_read_unlock();
+ wb = wb_get_lookup(bdi, memcg_css);
} while (!wb && !cgwb_create(bdi, memcg_css, gfp));

return wb;