[PATCH 3/4] writeback, memcg: Implement cgroup_writeback_by_id()

[Tejun Heo]

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 <tj@xxxxxxxxxx>
---
 fs/fs-writeback.c         | 64 +++++++++++++++++++++++++++++++++++++++
 include/linux/writeback.h |  4 +++
 2 files changed, 68 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6129debdc938..5c79d7acefdb 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -880,6 +880,70 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 		wb_put(last_wb);
 }
 
+/**
+ * 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 */
+	wb = wb_get_create(bdi, memcg_css, GFP_NOWAIT | __GFP_NOWARN);
+	if (!wb) {
+		ret = -ENOMEM;
+		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
  *
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 8945aac31392..ad794f2a7d42 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -217,6 +217,10 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
 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);
+int writeback_by_id(int id, unsigned long nr, enum wb_reason reason,
+		    struct wb_completion *done);
 void cgroup_writeback_umount(void);
 
 /**
-- 
2.17.1