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Subject[PATCH 15/16] mm: Throttle direct reclaimers if PF_MEMALLOC reserves are low and swap is backed by network storage
Date
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>
---
include/linux/mmzone.h | 1 +
mm/page_alloc.c | 1 +
mm/vmscan.c | 109 ++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 111 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index dff7115..e6b733d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -663,6 +663,7 @@ typedef struct pglist_data {
range, including holes */
int node_id;
wait_queue_head_t kswapd_wait;
+ wait_queue_head_t pfmemalloc_wait;
struct task_struct *kswapd;
int kswapd_max_order;
enum zone_type classzone_idx;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0d7bea5..71802cb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4325,6 +4325,7 @@ 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 33c332b..fdb63db 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2431,6 +2431,73 @@ 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) ? true : false;
+
+ /* 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 such as kjournald should not be throttled */
+ 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 processing a journal transaction. In this case, it is not safe
+ * to block on pfmemalloc_wait as kswapd could also be blocked waiting
+ * to start a transaction. Instead, throttle for up to a second before
+ * the reclaim must continue.
+ */
+ 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)
{
@@ -2449,6 +2516,15 @@ 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);
@@ -2610,6 +2686,20 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
if (remaining)
return true;

+ /*
+ * There is a potential race between when kswapd checks it 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 true;
+ }
+
/* Check the watermark levels */
for (i = 0; i <= classzone_idx; i++) {
struct zone *zone = pgdat->node_zones + i;
@@ -2871,6 +2961,12 @@ loop_again:
}

}
+
+ /* Wake throttled direct reclaimers if low watermark is met */
+ 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 */
/*
@@ -3005,6 +3101,19 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
trace_mm_vmscan_kswapd_sleep(pgdat->node_id);

/*
+ * There is a potential race between when kswapd checks it
+ * 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);
+
+ /*
* vmstat counters are not perfectly accurate and the estimated
* value for counters such as NR_FREE_PAGES can deviate from the
* true value by nr_online_cpus * threshold. To avoid the zone
--
1.7.9.2


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