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Subject[PATCH v3 4/5] writeback, memcg: Implement cgroup_writeback_by_id()
Implement cgroup_writeback_by_id() which initiates cgroup writeback
from bdi and memcg IDs. This will be used by memcg foreign inode
flushing.

v2: Use wb_get_lookup() instead of wb_get_create() to avoid creating
spurious wbs.

v3: Interpret 0 @nr as 1.25 * nr_dirty to implement best-effort
flushing while avoding possible livelocks.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
fs/fs-writeback.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++
include/linux/writeback.h | 2 +
2 files changed, 85 insertions(+)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -892,6 +892,89 @@ 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, 0 for best-effort dirty flushing
+ * @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;
+ }
+
+ /*
+ * If @nr is zero, the caller is attempting to write out most of
+ * the currently dirty pages. Let's take the current dirty page
+ * count and inflate it by 25% which should be large enough to
+ * flush out most dirty pages while avoiding getting livelocked by
+ * concurrent dirtiers.
+ */
+ if (!nr) {
+ unsigned long filepages, headroom, dirty, writeback;
+
+ mem_cgroup_wb_stats(wb, &filepages, &headroom, &dirty,
+ &writeback);
+ nr = dirty * 10 / 8;
+ }
+
+ /* 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->range_cyclic = 1;
+ 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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