mm/writeback: discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead

After an NFS page has been written it is considered "unstable" until a
COMMIT request succeeds.  If the COMMIT fails, the page will be
re-written.

These "unstable" pages are currently accounted as "reclaimable", either
in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
'reclaimable' count.  This might have made sense when sending the COMMIT
required a separate action by the VFS/MM (e.g.  releasepage() used to
send a COMMIT).  However now that all writes generated by ->writepages()
will automatically be followed by a COMMIT (since commit 919e3bd9a875
("NFS: Ensure we commit after writeback is complete")) it makes more
sense to treat them as writeback pages.

So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
NR_WRITEBACK and WB_WRITEBACK.

A particular effect of this change is that when
wb_check_background_flush() calls wb_over_bg_threshold(), the latter
will report 'true' a lot less often as the 'unstable' pages are no
longer considered 'dirty' (as there is nothing that writeback can do
about them anyway).

Currently wb_check_background_flush() will trigger writeback to NFS even
when there are relatively few dirty pages (if there are lots of unstable
pages), this can result in small writes going to the server (10s of
Kilobytes rather than a Megabyte) which hurts throughput.  With this
patch, there are fewer writes which are each larger on average.

Where the NR_UNSTABLE_NFS count was included in statistics
virtual-files, the entry is retained, but the value is hard-coded as
zero.  static trace points and warning printks which mentioned this
counter no longer report it.

[akpm@linux-foundation.org: re-layout comment]
[akpm@linux-foundation.org: fix printk warning]
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Acked-by: Michal Hocko <mhocko@suse.com>	[mm]
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chuck Lever <chuck.lever@oracle.com>
Link: http://lkml.kernel.org/r/87d06j7gqa.fsf@notabene.neil.brown.name
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 14 insertions, 20 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a3b97f103966..1db4b285c407 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4330,7 +4330,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 
 	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
 
-	/* this should eventually include NR_UNSTABLE_NFS */
 	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
 	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
 			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7ff2290cf43d..718565266257 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
 	unsigned long nr_pages = 0;
 
 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
 	return nr_pages <= limit;
@@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  *
  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
- * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
+ * dirty balancing includes all PG_dirty and PG_writeback pages.
  */
 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 {
@@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
 						     &mdtc_stor : NULL;
 	struct dirty_throttle_control *sdtc;
-	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
+	unsigned long nr_reclaimable;	/* = file_dirty */
 	long period;
 	long pause;
 	long max_pause;
@@ -1586,14 +1585,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		unsigned long m_thresh = 0;
 		unsigned long m_bg_thresh = 0;
 
-		/*
-		 * Unstable writes are a feature of certain networked
-		 * filesystems (i.e. NFS) in which data may have been
-		 * written to the server's write cache, but has not yet
-		 * been flushed to permanent storage.
-		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
@@ -1963,8 +1955,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	 * as we're trying to decide whether to put more under writeback.
 	 */
 	gdtc->avail = global_dirtyable_memory();
-	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-		      global_node_page_state(NR_UNSTABLE_NFS);
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
 	domain_dirty_limits(gdtc);
 
 	if (gdtc->dirty > gdtc->bg_thresh)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 13cc653122b7..cc406ee17ad9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5319,7 +5319,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 
 	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
 		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
-		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
+		" unevictable:%lu dirty:%lu writeback:%lu\n"
 		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
 		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
 		" free:%lu free_pcp:%lu free_cma:%lu\n",
@@ -5332,7 +5332,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 		global_node_page_state(NR_UNEVICTABLE),
 		global_node_page_state(NR_FILE_DIRTY),
 		global_node_page_state(NR_WRITEBACK),
-		global_node_page_state(NR_UNSTABLE_NFS),
 		global_node_page_state(NR_SLAB_RECLAIMABLE),
 		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
 		global_node_page_state(NR_FILE_MAPPED),
@@ -5365,7 +5364,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			" anon_thp: %lukB"
 #endif
 			" writeback_tmp:%lukB"
-			" unstable:%lukB"
 			" all_unreclaimable? %s"
 			"\n",
 			pgdat->node_id,
@@ -5387,7 +5385,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
 #endif
 			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
-			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
 			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
 				"yes" : "no");
 	}
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 96d21a792b57..b1582fdf757c 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
 					TEXT_FOR_HIGHMEM(xx) xx "_movable",
 
 const char * const vmstat_text[] = {
-	/* enum zone_stat_item countes */
+	/* enum zone_stat_item counters */
 	"nr_free_pages",
 	"nr_zone_inactive_anon",
 	"nr_zone_active_anon",
@@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
 	"nr_file_hugepages",
 	"nr_file_pmdmapped",
 	"nr_anon_transparent_hugepages",
-	"nr_unstable",
 	"nr_vmscan_write",
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
@@ -1723,6 +1722,14 @@ static int vmstat_show(struct seq_file *m, void *arg)
 	seq_puts(m, vmstat_text[off]);
 	seq_put_decimal_ull(m, " ", *l);
 	seq_putc(m, '\n');
+
+	if (off == NR_VMSTAT_ITEMS - 1) {
+		/*
+		 * We've come to the end - add any deprecated counters to avoid
+		 * breaking userspace which might depend on them being present.
+		 */
+		seq_puts(m, "nr_unstable 0\n");
+	}
 	return 0;
 }