From d9ae4243d5abc7c0ca9fb44a073f8d50450f435c Mon Sep 17 00:00:00 2001

From: Jens Axboe <axboe@kernel.dk>

Date: Thu, 28 Sep 2017 11:31:55 -0600

Subject: [PATCH] writeback: only allow one inflight and pending full flush

References: bnc#1081213

Patch-mainline: v4.15-rc1

Git-commit: aac8d41cd438f25bf3110fc6b98f1d16d7dbc169

When someone calls wakeup_flusher_threads() or

wakeup_flusher_threads_bdi(), they schedule writeback of all dirty

pages in the system (or on that bdi). 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.

   We've seen as much as 600 million of these writeback work items

   pending. That's a lot of memory to pointlessly hold hostage,

   while the box is under memory pressure.

2) We spend so much time processing these work items, that we

   introduce a softlockup in writeback processing. This is because

   each of the writeback work items don't end up doing any work (it's

   hard when you have millions of identical ones coming in to the

   flush machinery), so we just sit in a tight loop pulling work

   items and deleting/freeing them.

Fix this by adding a 'start_all' bit to the writeback structure, and

set that when someone attempts to flush all dirty pages. 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 full flush in flight, with one pending as well,

and makes for more efficient handling of this type of writeback.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Tested-by: Chris Mason <clm@fb.com>

Reviewed-by: Jan Kara <jack@suse.cz>

Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Mel Gorman <mgorman@suse.de>

---

 fs/fs-writeback.c                | 25 +++++++++++++++++++++++++

 include/linux/backing-dev-defs.h |  1 +

 2 files changed, 26 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c

index e8799b0b9727..b33ca14d8531 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 start_all:1;	/* nr_pages == 0 (all) writeback */

 	enum wb_reason reason;		/* why was writeback initiated? */

 	struct list_head list;		/* pending work list */

@@ -951,6 +952,20 @@ static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)

 	if (!wb_has_dirty_io(wb))

 		return;

+	/*

+	 * All callers of this function want to start writeback of all

+	 * dirty pages. Places like vmscan can call this at a very

+	 * high frequency, causing pointless allocations of tons of

+	 * work items and keeping the flusher threads busy retrieving

+	 * that work. Ensure that we only allow one of them pending and

+	 * inflight at the time. It doesn't matter if we race a little

+	 * bit on this, so use the faster separate test/set bit variants.

+	 */

+	if (test_bit(WB_start_all, &wb->state))

+		return;

+

+	set_bit(WB_start_all, &wb->state);

+

 	/*

 	 * This is WB_SYNC_NONE writeback, so if allocation fails just

 	 * wakeup the thread for old dirty data writeback

@@ -958,6 +973,7 @@ static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)

 	work = kzalloc(sizeof(*work),

 		       GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);

 	if (!work) {

+		clear_bit(WB_start_all, &wb->state);

 		trace_writeback_nowork(wb);

 		wb_wakeup(wb);

 		return;

@@ -968,6 +984,7 @@ static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)

 	work->range_cyclic = 1;

 	work->reason	= reason;

 	work->auto_free	= 1;

+	work->start_all = 1;

 	wb_queue_work(wb, work);

 }

@@ -1821,6 +1838,14 @@ 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->start_all)

+		clear_bit(WB_start_all, &wb->state);

+

 	return work;

 }

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h

index 866c433e7d32..420de5c7c7f9 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_start_all,		/* nr_pages == 0 (all) work pending */

 };

 enum wb_congested_state {