Implement cgroup_writeback_by_id() which initiates cgroup writeback
from bdi and memcg IDs.  This will be used by memcg foreign inode
flushing.

Signed-off-by: Tejun Heo <t...@kernel.org>
---
 fs/fs-writeback.c         |   67 ++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/writeback.h |    2 +
 2 files changed, 69 insertions(+)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -892,6 +892,73 @@ restart:
 }
 
 /**
+ * cgroup_writeback_by_id - initiate cgroup writeback from bdi and memcg IDs
+ * @bdi_id: target bdi id
+ * @memcg_id: target memcg css id
+ * @nr_pages: number of pages to write
+ * @reason: reason why some writeback work initiated
+ * @done: target wb_completion
+ *
+ * Initiate flush of the bdi_writeback identified by @bdi_id and @memcg_id
+ * with the specified parameters.
+ */
+int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr,
+                          enum wb_reason reason, struct wb_completion *done)
+{
+       struct backing_dev_info *bdi;
+       struct cgroup_subsys_state *memcg_css;
+       struct bdi_writeback *wb;
+       struct wb_writeback_work *work;
+       int ret;
+
+       /* lookup bdi and memcg */
+       bdi = bdi_get_by_id(bdi_id);
+       if (!bdi)
+               return -ENOENT;
+
+       rcu_read_lock();
+       memcg_css = css_from_id(memcg_id, &memory_cgrp_subsys);
+       if (memcg_css && !css_tryget(memcg_css))
+               memcg_css = NULL;
+       rcu_read_unlock();
+       if (!memcg_css) {
+               ret = -ENOENT;
+               goto out_bdi_put;
+       }
+
+       /*
+        * And find the associated wb.  If the wb isn't there already
+        * there's nothing to flush, don't create one.
+        */
+       wb = wb_get_lookup(bdi, memcg_css);
+       if (!wb) {
+               ret = -ENOENT;
+               goto out_css_put;
+       }
+
+       /* issue the writeback work */
+       work = kzalloc(sizeof(*work), GFP_NOWAIT | __GFP_NOWARN);
+       if (work) {
+               work->nr_pages = nr;
+               work->sync_mode = WB_SYNC_NONE;
+               work->reason = reason;
+               work->done = done;
+               work->auto_free = 1;
+               wb_queue_work(wb, work);
+               ret = 0;
+       } else {
+               ret = -ENOMEM;
+       }
+
+       wb_put(wb);
+out_css_put:
+       css_put(memcg_css);
+out_bdi_put:
+       bdi_put(bdi);
+       return ret;
+}
+
+/**
  * cgroup_writeback_umount - flush inode wb switches for umount
  *
  * This function is called when a super_block is about to be destroyed and
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -217,6 +217,8 @@ void wbc_attach_and_unlock_inode(struct
 void wbc_detach_inode(struct writeback_control *wbc);
 void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
                              size_t bytes);
+int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr_pages,
+                          enum wb_reason reason, struct wb_completion *done);
 void cgroup_writeback_umount(void);
 
 /**

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