Currently, balance_dirty_pages() always work on bdi->wb.  This patch
updates it to work on the wb (bdi_writeback) matching memcg and blkcg
of the current task as that's what the inode is being dirtied against.

balance_dirty_pages_ratelimited() now pins the current wb and passes
it to balance_dirty_pages().

As no filesystem has FS_CGROUP_WRITEBACK yet, this doesn't lead to
visible behavior differences.

v2: Updated for per-inode wb association.

Signed-off-by: Tejun Heo <t...@kernel.org>
Cc: Jens Axboe <ax...@kernel.dk>
Cc: Jan Kara <j...@suse.cz>
---
 mm/page-writeback.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3b6d058..0aa2ffe 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1337,6 +1337,7 @@ static inline void wb_dirty_limits(struct bdi_writeback 
*wb,
  * perform some writeout.
  */
 static void balance_dirty_pages(struct address_space *mapping,
+                               struct bdi_writeback *wb,
                                unsigned long pages_dirtied)
 {
        unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
@@ -1352,8 +1353,7 @@ static void balance_dirty_pages(struct address_space 
*mapping,
        unsigned long task_ratelimit;
        unsigned long dirty_ratelimit;
        unsigned long pos_ratio;
-       struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
-       struct bdi_writeback *wb = &bdi->wb;
+       struct backing_dev_info *bdi = wb->bdi;
        bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
        unsigned long start_time = jiffies;
 
@@ -1575,14 +1575,20 @@ DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
  */
 void balance_dirty_pages_ratelimited(struct address_space *mapping)
 {
-       struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
-       struct bdi_writeback *wb = &bdi->wb;
+       struct inode *inode = mapping->host;
+       struct backing_dev_info *bdi = inode_to_bdi(inode);
+       struct bdi_writeback *wb = NULL;
        int ratelimit;
        int *p;
 
        if (!bdi_cap_account_dirty(bdi))
                return;
 
+       if (inode_cgwb_enabled(inode))
+               wb = wb_get_create_current(bdi, GFP_KERNEL);
+       if (!wb)
+               wb = &bdi->wb;
+
        ratelimit = current->nr_dirtied_pause;
        if (wb->dirty_exceeded)
                ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
@@ -1616,7 +1622,9 @@ void balance_dirty_pages_ratelimited(struct address_space 
*mapping)
        preempt_enable();
 
        if (unlikely(current->nr_dirtied >= ratelimit))
-               balance_dirty_pages(mapping, current->nr_dirtied);
+               balance_dirty_pages(mapping, wb, current->nr_dirtied);
+
+       wb_put(wb);
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
 
-- 
2.1.0

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