[PATCH 1/2] mm: Make task in balance_dirty_pages() killable

[Jan Kara <jack@suse.cz>]

There is no reason why task in balance_dirty_pages() shouldn't be killable
and it helps in recovering from some error conditions (like when filesystem
goes in error state and cannot accept writeback anymore but we still want to
kill processes using it to be able to unmount it).

Reported-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Tested-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 include/linux/writeback.h |    8 ++++----
 mm/page-writeback.c       |   27 +++++++++++++++++++--------
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index a378c29..eb3ecca 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -170,13 +170,13 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
 			    unsigned long start_time);
 
 void page_writeback_init(void);
-void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
-					unsigned long nr_pages_dirtied);
+int balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
+				       unsigned long nr_pages_dirtied);
 
-static inline void
+static inline int
 balance_dirty_pages_ratelimited(struct address_space *mapping)
 {
-	balance_dirty_pages_ratelimited_nr(mapping, 1);
+	return balance_dirty_pages_ratelimited_nr(mapping, 1);
 }
 
 typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc,
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0360d1b..380279c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1000,8 +1000,11 @@ static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
  * If we're over `background_thresh' then the writeback threads are woken to
  * perform some writeout.
+ *
+ * We return 0 if nothing special happened, EINTR if our wait has been
+ * interrupted by a fatal signal.
  */
-static void balance_dirty_pages(struct address_space *mapping,
+static int balance_dirty_pages(struct address_space *mapping,
 				unsigned long pages_dirtied)
 {
 	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
@@ -1020,6 +1023,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 	unsigned long pos_ratio;
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 	unsigned long start_time = jiffies;
+	int err = 0;
 
 	for (;;) {
 		/*
@@ -1133,7 +1137,7 @@ pause:
 					  pages_dirtied,
 					  pause,
 					  start_time);
-		__set_current_state(TASK_UNINTERRUPTIBLE);
+		__set_current_state(TASK_KILLABLE);
 		io_schedule_timeout(pause);
 
 		dirty_thresh = hard_dirty_limit(dirty_thresh);
@@ -1145,6 +1149,11 @@ pause:
 		 */
 		if (nr_dirty < dirty_thresh)
 			break;
+
+		if (fatal_signal_pending(current)) {
+			err = -EINTR;
+			break;
+		}
 	}
 
 	if (!dirty_exceeded && bdi->dirty_exceeded)
@@ -1168,7 +1177,7 @@ pause:
 	}
 
 	if (writeback_in_progress(bdi))
-		return;
+		return err;
 
 	/*
 	 * In laptop mode, we wait until hitting the higher threshold before
@@ -1179,10 +1188,11 @@ pause:
 	 * background_thresh, to keep the amount of dirty memory low.
 	 */
 	if (laptop_mode)
-		return;
+		return err;
 
 	if (nr_reclaimable > background_thresh)
 		bdi_start_background_writeback(bdi);
+	return err;
 }
 
 void set_page_dirty_balance(struct page *page, int page_mkwrite)
@@ -1211,15 +1221,15 @@ static DEFINE_PER_CPU(int, bdp_ratelimits);
  * limit we decrease the ratelimiting by a lot, to prevent individual processes
  * from overshooting the limit by (ratelimit_pages) each.
  */
-void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
-					unsigned long nr_pages_dirtied)
+int balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
+				       unsigned long nr_pages_dirtied)
 {
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 	int ratelimit;
 	int *p;
 
 	if (!bdi_cap_account_dirty(bdi))
-		return;
+		return 0;
 
 	ratelimit = current->nr_dirtied_pause;
 	if (bdi->dirty_exceeded)
@@ -1247,7 +1257,8 @@ void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
 	preempt_enable();
 
 	if (unlikely(current->nr_dirtied >= ratelimit))
-		balance_dirty_pages(mapping, current->nr_dirtied);
+		return balance_dirty_pages(mapping, current->nr_dirtied);
+	return 0;
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);
 
-- 
1.7.1