+ writeback-fix-bandwidth-estimate-for-spiky-workload-fix.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: writeback: avoid division by 0 in wb_update_dirty_ratelimit()
has been added to the -mm tree.  Its filename is
     writeback-fix-bandwidth-estimate-for-spiky-workload-fix.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/writeback-fix-bandwidth-estimate-for-spiky-workload-fix.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/writeback-fix-bandwidth-estimate-for-spiky-workload-fix.patch

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------------------------------------------------------
From: Jan Kara <jack@suse.cz>
Subject: writeback: avoid division by 0 in wb_update_dirty_ratelimit()

Fixup patch "writeback: Fix bandwidth estimate for spiky workload" which
introduced possibility of __wb_update_bandwidth() getting called at a
moment when 'elapsed' evaluates to 0.

Cc: Jan Kara <jack@suse.cz>
Cc: Michael Stapelberg <stapelberg+linux@google.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page-writeback.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/mm/page-writeback.c~writeback-fix-bandwidth-estimate-for-spiky-workload-fix
+++ a/mm/page-writeback.c
@@ -1336,11 +1336,19 @@ static void __wb_update_bandwidth(struct
 {
 	struct bdi_writeback *wb = gdtc->wb;
 	unsigned long now = jiffies;
-	unsigned long elapsed = now - wb->bw_time_stamp;
+	unsigned long elapsed;
 	unsigned long dirtied;
 	unsigned long written;
 
 	spin_lock(&wb->list_lock);
+
+	/*
+	 * Lockless checks for elapsed time are racy and delayed update after
+	 * IO completion doesn't do it at all (to make sure written pages are
+	 * accounted reasonably quickly). Make sure elapsed >= 1 to avoid
+	 * division errors.
+	 */
+	elapsed = max(now - wb->bw_time_stamp, 1UL);
 	dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
 	written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
 
_

Patches currently in -mm which might be from jack@suse.cz are

writeback-track-number-of-inodes-under-writeback.patch
writeback-reliably-update-bandwidth-estimation.patch
writeback-fix-bandwidth-estimate-for-spiky-workload.patch
writeback-fix-bandwidth-estimate-for-spiky-workload-fix.patch
writeback-rename-domain_update_bandwidth.patch
writeback-use-read_once-for-unlocked-reads-of-writeback-stats.patch