[PATCH 6/6] fs-writeback: only allow one inflight and pending !nr_pages flush

[Jens Axboe]

A few callers pass in nr_pages == 0 when they wakeup the flusher
threads, which means that the flusher should just flush everything
that was currently dirty. If we are tight on memory, we can get
tons of these queued from kswapd/vmscan. This causes (at least)
two problems:

1) We consume a ton of memory just allocating writeback work items.
2) We spend so much time processing these work items, that we
   introduce a softlockup in writeback processing.

Fix this by adding a 'zero_pages' bit to the writeback structure,
and set that when someone queues a nr_pages==0 flusher thread
wakeup. The bit is cleared when we start writeback on that work
item. If the bit is already set when we attempt to queue !nr_pages
writeback, then we simply ignore it.

This provides us one of full flush in flight, with one pending as
well, and makes for more efficient handling of this type of
writeback.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 fs/fs-writeback.c                | 30 ++++++++++++++++++++++++++++--
 include/linux/backing-dev-defs.h |  1 +
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a9a86644cb9f..e0240110b36f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -53,6 +53,7 @@ struct wb_writeback_work {
 	unsigned int for_background:1;
 	unsigned int for_sync:1;	/* sync(2) WB_SYNC_ALL writeback */
 	unsigned int auto_free:1;	/* free on completion */
+	unsigned int zero_pages:1;	/* nr_pages == 0 writeback */
 	enum wb_reason reason;		/* why was writeback initiated? */
 
 	struct list_head list;		/* pending work list */
@@ -948,15 +949,25 @@ static void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
 			       bool range_cyclic, enum wb_reason reason)
 {
 	struct wb_writeback_work *work;
+	bool zero_pages = false;
 
 	if (!wb_has_dirty_io(wb))
 		return;
 
 	/*
-	 * If someone asked for zero pages, we write out the WORLD
+	 * If someone asked for zero pages, we write out the WORLD.
+	 * Places like vmscan and laptop mode want to queue a wakeup to
+	 * the flusher threads to clean out everything. To avoid potentially
+	 * having tons of these pending, ensure that we only allow one of
+	 * them pending and inflight at the time
 	 */
-	if (!nr_pages)
+	if (!nr_pages) {
+		if (test_bit(WB_zero_pages, &wb->state))
+			return;
+		set_bit(WB_zero_pages, &wb->state);
 		nr_pages = get_nr_dirty_pages();
+		zero_pages = true;
+	}
 
 	/*
 	 * This is WB_SYNC_NONE writeback, so if allocation fails just
@@ -975,6 +986,7 @@ static void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
 	work->range_cyclic = range_cyclic;
 	work->reason	= reason;
 	work->auto_free	= 1;
+	work->zero_pages = zero_pages;
 
 	wb_queue_work(wb, work);
 }
@@ -1828,6 +1840,14 @@ static struct wb_writeback_work *get_next_work_item(struct bdi_writeback *wb)
 		list_del_init(&work->list);
 	}
 	spin_unlock_bh(&wb->work_lock);
+
+	/*
+	 * Once we start processing a work item that had !nr_pages,
+	 * clear the wb state bit for that so we can allow more.
+	 */
+	if (work && work->zero_pages && test_bit(WB_zero_pages, &wb->state))
+		clear_bit(WB_zero_pages, &wb->state);
+
 	return work;
 }
 
@@ -1896,6 +1916,12 @@ static long wb_do_writeback(struct bdi_writeback *wb)
 		trace_writeback_exec(wb, work);
 		wrote += wb_writeback(wb, work);
 		finish_writeback_work(wb, work);
+
+		/*
+		 * If we have a lot of pending work, make sure we take
+		 * an occasional breather, if needed.
+		 */
+		cond_resched();
 	}
 
 	/*
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 866c433e7d32..7494f6a75458 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -24,6 +24,7 @@ enum wb_state {
 	WB_shutting_down,	/* wb_shutdown() in progress */
 	WB_writeback_running,	/* Writeback is in progress */
 	WB_has_dirty_io,	/* Dirty inodes on ->b_{dirty|io|more_io} */
+	WB_zero_pages,		/* nr_pages == 0 flush pending */
 };
 
 enum wb_congested_state {
-- 
2.7.4