There currently is no way to universally identify and lookup a bdi
without holding a reference and pointer to it.  This patch adds an
non-recycling bdi->id and implements bdi_get_by_id() which looks up
bdis by their ids.  This will be used by memcg foreign inode flushing.

I left bdi_list alone for simplicity and because while rb_tree does
support rcu assignment it doesn't seem to guarantee lossless walk when
walk is racing aginst tree rebalance operations.

Signed-off-by: Tejun Heo <t...@kernel.org>
Reviewed-by: Jan Kara <j...@suse.cz>
---
 include/linux/backing-dev-defs.h |    2 +
 include/linux/backing-dev.h      |    1 
 mm/backing-dev.c                 |   65 +++++++++++++++++++++++++++++++++++++--
 3 files changed, 66 insertions(+), 2 deletions(-)

--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -185,6 +185,8 @@ struct bdi_writeback {
 };
 
 struct backing_dev_info {
+       u64 id;
+       struct rb_node rb_node; /* keyed by ->id */
        struct list_head bdi_list;
        unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
        unsigned long io_pages; /* max allowed IO size */
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -24,6 +24,7 @@ static inline struct backing_dev_info *b
        return bdi;
 }
 
+struct backing_dev_info *bdi_get_by_id(u64 id);
 void bdi_put(struct backing_dev_info *bdi);
 
 __printf(2, 3)
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
 
 #include <linux/wait.h>
+#include <linux/rbtree.h>
 #include <linux/backing-dev.h>
 #include <linux/kthread.h>
 #include <linux/freezer.h>
@@ -22,10 +23,12 @@ EXPORT_SYMBOL_GPL(noop_backing_dev_info)
 static struct class *bdi_class;
 
 /*
- * bdi_lock protects updates to bdi_list. bdi_list has RCU reader side
- * locking.
+ * bdi_lock protects bdi_tree and updates to bdi_list. bdi_list has RCU
+ * reader side locking.
  */
 DEFINE_SPINLOCK(bdi_lock);
+static u64 bdi_id_cursor;
+static struct rb_root bdi_tree = RB_ROOT;
 LIST_HEAD(bdi_list);
 
 /* bdi_wq serves all asynchronous writeback tasks */
@@ -859,9 +862,58 @@ struct backing_dev_info *bdi_alloc_node(
 }
 EXPORT_SYMBOL(bdi_alloc_node);
 
+static struct rb_node **bdi_lookup_rb_node(u64 id, struct rb_node **parentp)
+{
+       struct rb_node **p = &bdi_tree.rb_node;
+       struct rb_node *parent = NULL;
+       struct backing_dev_info *bdi;
+
+       lockdep_assert_held(&bdi_lock);
+
+       while (*p) {
+               parent = *p;
+               bdi = rb_entry(parent, struct backing_dev_info, rb_node);
+
+               if (bdi->id > id)
+                       p = &(*p)->rb_left;
+               else if (bdi->id < id)
+                       p = &(*p)->rb_right;
+               else
+                       break;
+       }
+
+       if (parentp)
+               *parentp = parent;
+       return p;
+}
+
+/**
+ * bdi_get_by_id - lookup and get bdi from its id
+ * @id: bdi id to lookup
+ *
+ * Find bdi matching @id and get it.  Returns NULL if the matching bdi
+ * doesn't exist or is already unregistered.
+ */
+struct backing_dev_info *bdi_get_by_id(u64 id)
+{
+       struct backing_dev_info *bdi = NULL;
+       struct rb_node **p;
+
+       spin_lock_bh(&bdi_lock);
+       p = bdi_lookup_rb_node(id, NULL);
+       if (*p) {
+               bdi = rb_entry(*p, struct backing_dev_info, rb_node);
+               bdi_get(bdi);
+       }
+       spin_unlock_bh(&bdi_lock);
+
+       return bdi;
+}
+
 int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list 
args)
 {
        struct device *dev;
+       struct rb_node *parent, **p;
 
        if (bdi->dev)   /* The driver needs to use separate queues per device */
                return 0;
@@ -877,7 +929,15 @@ int bdi_register_va(struct backing_dev_i
        set_bit(WB_registered, &bdi->wb.state);
 
        spin_lock_bh(&bdi_lock);
+
+       bdi->id = ++bdi_id_cursor;
+
+       p = bdi_lookup_rb_node(bdi->id, &parent);
+       rb_link_node(&bdi->rb_node, parent, p);
+       rb_insert_color(&bdi->rb_node, &bdi_tree);
+
        list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
+
        spin_unlock_bh(&bdi_lock);
 
        trace_writeback_bdi_register(bdi);
@@ -918,6 +978,7 @@ EXPORT_SYMBOL(bdi_register_owner);
 static void bdi_remove_from_list(struct backing_dev_info *bdi)
 {
        spin_lock_bh(&bdi_lock);
+       rb_erase(&bdi->rb_node, &bdi_tree);
        list_del_rcu(&bdi->bdi_list);
        spin_unlock_bh(&bdi_lock);
 

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