Foreign dirty tracking records the bdi and wb memcg IDs of recently
dirtied foreign inodes. When the source memcg needs foreign writeback,
mem_cgroup_flush_foreign() queues WB_REASON_FOREIGN_FLUSH to the
recorded target bdi_writeback.

cgroup_writeback_by_id() currently sizes this best-effort work from the
target memcg's NR_FILE_DIRTY counter. The work is scoped to one target
wb, so a memcg-wide dirty count can over-size a single-bdi flush when
the target memcg has dirty pages on other devices. It can also under-size
the flush when the target wb has dirty pages charged to other memcgs.

Over-sizing keeps the target wb busy longer and can delay later
writeback work. Under-sizing can finish before enough pages are written
back for the source memcg.

Use the target wb's WB_RECLAIMABLE counter and keep the existing 25%
headroom. This keeps the budget aligned with the writeback object being
queued.

Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Xin Yin <yinxin.x@bytedance.com>
---
Changes in v2:
- Submit the budget fix on its own. Drop the snapshot writeback patch
  for now, pending further evidence of its benefit in production.
- Add Jan's Reviewed-by and Tejun's Acked-by.
- No code changes from v1 patch 1.

v1: https://lore.kernel.org/linux-fsdevel/cover.1788836830.git.yinxin.x@bytedance.com/

 fs/fs-writeback.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 7c75ed7e8979..7c2340a5dead 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1131,7 +1131,7 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
 	struct cgroup_subsys_state *memcg_css;
 	struct bdi_writeback *wb;
 	struct wb_writeback_work *work;
-	unsigned long dirty;
+	long dirty;
 	int ret;
 
 	/* lookup bdi and memcg */
@@ -1160,16 +1160,13 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
 	}
 
 	/*
-	 * 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.
-	 *
-	 * BTW the memcg stats are flushed periodically and this is best-effort
-	 * estimation, so some potential error is ok.
+	 * The caller is attempting to write out most of the target wb's
+	 * currently dirty pages.  Size the work from the wb's reclaimable pages
+	 * and inflate the count by 25%, which should be large enough to flush
+	 * out most dirty pages while avoiding getting livelocked by concurrent
+	 * dirtiers.
 	 */
-	dirty = memcg_page_state(mem_cgroup_from_css(memcg_css), NR_FILE_DIRTY);
+	dirty = wb_stat_sum(wb, WB_RECLAIMABLE);
 	dirty = dirty * 10 / 8;
 
 	/* issue the writeback work */

base-commit: 46b513250491a7bfc97d98791dbe6a10bcc8129d
-- 
2.20.1