mm: throttle direct reclaimers if PF_MEMALLOC reserves are low and swap is backed by network storage

If swap is backed by network storage such as NBD, there is a risk that a
large number of reclaimers can hang the system by consuming all
PF_MEMALLOC reserves.  To avoid these hangs, the administrator must tune
min_free_kbytes in advance which is a bit fragile.

This patch throttles direct reclaimers if half the PF_MEMALLOC reserves
are in use.  If the system is routinely getting throttled the system
administrator can increase min_free_kbytes so degradation is smoother but
the system will keep running.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: David Miller <davem@davemloft.net>
Cc: Neil Brown <neilb@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Cc: Eric B Munson <emunson@mgebm.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 121 insertions, 8 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ef2d1e72fc07..48aee0f5902f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4389,6 +4389,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
         pgdat_resize_init(pgdat);
         pgdat->nr_zones = 0;
         init_waitqueue_head(&pgdat->kswapd_wait);
+        init_waitqueue_head(&pgdat->pfmemalloc_wait);
         pgdat->kswapd_max_order = 0;
         pgdat_page_cgroup_init(pgdat);
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6b1f89a91212..021a44a7bd20 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2112,6 +2112,80 @@ out:
         return 0;
 }
 
+static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
+{
+        struct zone *zone;
+        unsigned long pfmemalloc_reserve = 0;
+        unsigned long free_pages = 0;
+        int i;
+        bool wmark_ok;
+
+        for (i = 0; i <= ZONE_NORMAL; i++) {
+                zone = &pgdat->node_zones[i];
+                pfmemalloc_reserve += min_wmark_pages(zone);
+                free_pages += zone_page_state(zone, NR_FREE_PAGES);
+        }
+
+        wmark_ok = free_pages > pfmemalloc_reserve / 2;
+
+        /* kswapd must be awake if processes are being throttled */
+        if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
+                pgdat->classzone_idx = min(pgdat->classzone_idx,
+                                                (enum zone_type)ZONE_NORMAL);
+                wake_up_interruptible(&pgdat->kswapd_wait);
+        }
+
+        return wmark_ok;
+}
+
+/*
+ * Throttle direct reclaimers if backing storage is backed by the network
+ * and the PFMEMALLOC reserve for the preferred node is getting dangerously
+ * depleted. kswapd will continue to make progress and wake the processes
+ * when the low watermark is reached
+ */
+static void throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
+                                        nodemask_t *nodemask)
+{
+        struct zone *zone;
+        int high_zoneidx = gfp_zone(gfp_mask);
+        pg_data_t *pgdat;
+
+        /*
+         * Kernel threads should not be throttled as they may be indirectly
+         * responsible for cleaning pages necessary for reclaim to make forward
+         * progress. kjournald for example may enter direct reclaim while
+         * committing a transaction where throttling it could forcing other
+         * processes to block on log_wait_commit().
+         */
+        if (current->flags & PF_KTHREAD)
+                return;
+
+        /* Check if the pfmemalloc reserves are ok */
+        first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);
+        pgdat = zone->zone_pgdat;
+        if (pfmemalloc_watermark_ok(pgdat))
+                return;
+
+        /*
+         * If the caller cannot enter the filesystem, it's possible that it
+         * is due to the caller holding an FS lock or performing a journal
+         * transaction in the case of a filesystem like ext[3|4]. In this case,
+         * it is not safe to block on pfmemalloc_wait as kswapd could be
+         * blocked waiting on the same lock. Instead, throttle for up to a
+         * second before continuing.
+         */
+        if (!(gfp_mask & __GFP_FS)) {
+                wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
+                        pfmemalloc_watermark_ok(pgdat), HZ);
+                return;
+        }
+
+        /* Throttle until kswapd wakes the process */
+        wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
+                pfmemalloc_watermark_ok(pgdat));
+}
+
 unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
                                 gfp_t gfp_mask, nodemask_t *nodemask)
 {
@@ -2131,6 +2205,15 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
                 .gfp_mask = sc.gfp_mask,
         };
 
+        throttle_direct_reclaim(gfp_mask, zonelist, nodemask);
+
+        /*
+         * Do not enter reclaim if fatal signal is pending. 1 is returned so
+         * that the page allocator does not consider triggering OOM
+         */
+        if (fatal_signal_pending(current))
+                return 1;
+
         trace_mm_vmscan_direct_reclaim_begin(order,
                                 sc.may_writepage,
                                 gfp_mask);
@@ -2275,8 +2358,13 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
         return balanced_pages >= (present_pages >> 2);
 }
 
-/* is kswapd sleeping prematurely? */
-static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
+/*
+ * Prepare kswapd for sleeping. This verifies that there are no processes
+ * waiting in throttle_direct_reclaim() and that watermarks have been met.
+ *
+ * Returns true if kswapd is ready to sleep
+ */
+static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
                                         int classzone_idx)
 {
         int i;
@@ -2285,7 +2373,21 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 
         /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
         if (remaining)
-                return true;
+                return false;
+
+        /*
+         * There is a potential race between when kswapd checks its watermarks
+         * and a process gets throttled. There is also a potential race if
+         * processes get throttled, kswapd wakes, a large process exits therby
+         * balancing the zones that causes kswapd to miss a wakeup. If kswapd
+         * is going to sleep, no process should be sleeping on pfmemalloc_wait
+         * so wake them now if necessary. If necessary, processes will wake
+         * kswapd and get throttled again
+         */
+        if (waitqueue_active(&pgdat->pfmemalloc_wait)) {
+                wake_up(&pgdat->pfmemalloc_wait);
+                return false;
+        }
 
         /* Check the watermark levels */
         for (i = 0; i <= classzone_idx; i++) {
@@ -2318,9 +2420,9 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
          * must be balanced
          */
         if (order)
-                return !pgdat_balanced(pgdat, balanced, classzone_idx);
+                return pgdat_balanced(pgdat, balanced, classzone_idx);
         else
-                return !all_zones_ok;
+                return all_zones_ok;
 }
 
 /*
@@ -2546,6 +2648,16 @@ loop_again:
                         }
 
                 }
+
+                /*
+                 * If the low watermark is met there is no need for processes
+                 * to be throttled on pfmemalloc_wait as they should not be
+                 * able to safely make forward progress. Wake them
+                 */
+                if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
+                                pfmemalloc_watermark_ok(pgdat))
+                        wake_up(&pgdat->pfmemalloc_wait);
+
                 if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
                         break;          /* kswapd: all done */
                 /*
@@ -2647,7 +2759,7 @@ out:
         }
 
         /*
-         * Return the order we were reclaiming at so sleeping_prematurely()
+         * Return the order we were reclaiming at so prepare_kswapd_sleep()
          * makes a decision on the order we were last reclaiming at. However,
          * if another caller entered the allocator slow path while kswapd
          * was awake, order will remain at the higher level
@@ -2667,7 +2779,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
         prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 
         /* Try to sleep for a short interval */
-        if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {
+        if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
                 remaining = schedule_timeout(HZ/10);
                 finish_wait(&pgdat->kswapd_wait, &wait);
                 prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
@@ -2677,7 +2789,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
          * After a short sleep, check if it was a premature sleep. If not, then
          * go fully to sleep until explicitly woken up.
          */
-        if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {
+        if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
                 trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
 
                 /*