This is a note to let you know that I've just added the patch titled

mm: fix 100% CPU kswapd busyloop on unreclaimable nodes

to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mm-fix-100-cpu-kswapd-busyloop-on-unreclaimable-nodes.patch and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From c73322d098e4b6f5f0f0fa1330bf57e218775539 Mon Sep 17 00:00:00 2001

From: Johannes Weiner hannes@cmpxchg.org Date: Wed, 3 May 2017 14:51:51 -0700 Subject: mm: fix 100% CPU kswapd busyloop on unreclaimable nodes

From: Johannes Weiner hannes@cmpxchg.org

commit c73322d098e4b6f5f0f0fa1330bf57e218775539 upstream.

Patch series "mm: kswapd spinning on unreclaimable nodes - fixes and cleanups".

Jia reported a scenario in which the kswapd of a node indefinitely spins at 100% CPU usage. We have seen similar cases at Facebook.

The kernel's current method of judging its ability to reclaim a node (or whether to back off and sleep) is based on the amount of scanned pages in proportion to the amount of reclaimable pages. In Jia's and our scenarios, there are no reclaimable pages in the node, however, and the condition for backing off is never met. Kswapd busyloops in an attempt to restore the watermarks while having nothing to work with.

This series reworks the definition of an unreclaimable node based not on scanning but on whether kswapd is able to actually reclaim pages in MAX_RECLAIM_RETRIES (16) consecutive runs. This is the same criteria the page allocator uses for giving up on direct reclaim and invoking the OOM killer. If it cannot free any pages, kswapd will go to sleep and leave further attempts to direct reclaim invocations, which will either make progress and re-enable kswapd, or invoke the OOM killer.

Patch #1 fixes the immediate problem Jia reported, the remainder are smaller fixlets, cleanups, and overall phasing out of the old method.

Patch #6 is the odd one out. It's a nice cleanup to get_scan_count(), and directly related to #5, but in itself not relevant to the series.

If the whole series is too ambitious for 4.11, I would consider the first three patches fixes, the rest cleanups.

This patch (of 9):

Jia He reports a problem with kswapd spinning at 100% CPU when requesting more hugepages than memory available in the system:

$ echo 4000 >/proc/sys/vm/nr_hugepages

top - 13:42:59 up 3:37, 1 user, load average: 1.09, 1.03, 1.01 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.0 us, 12.5 sy, 0.0 ni, 85.5 id, 2.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem: 31371520 total, 30915136 used, 456384 free, 320 buffers KiB Swap: 6284224 total, 115712 used, 6168512 free. 48192 cached Mem

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 76 root 20 0 0 0 0 R 100.0 0.000 217:17.29 kswapd3

At that time, there are no reclaimable pages left in the node, but as kswapd fails to restore the high watermarks it refuses to go to sleep.

Kswapd needs to back away from nodes that fail to balance. Up until commit 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes") kswapd had such a mechanism. It considered zones whose theoretically reclaimable pages it had reclaimed six times over as unreclaimable and backed away from them. This guard was erroneously removed as the patch changed the definition of a balanced node.

However, simply restoring this code wouldn't help in the case reported here: there *are* no reclaimable pages that could be scanned until the threshold is met. Kswapd would stay awake anyway.

Introduce a new and much simpler way of backing off. If kswapd runs through MAX_RECLAIM_RETRIES (16) cycles without reclaiming a single page, make it back off from the node. This is the same number of shots direct reclaim takes before declaring OOM. Kswapd will go to sleep on that node until a direct reclaimer manages to reclaim some pages, thus proving the node reclaimable again.

[hannes@cmpxchg.org: check kswapd failure against the cumulative nr_reclaimed count] Link: http://lkml.kernel.org/r/20170306162410.GB2090@cmpxchg.org [shakeelb@google.com: fix condition for throttle_direct_reclaim] Link: http://lkml.kernel.org/r/20170314183228.20152-1-shakeelb@google.com Link: http://lkml.kernel.org/r/20170228214007.5621-2-hannes@cmpxchg.org Signed-off-by: Johannes Weiner hannes@cmpxchg.org Signed-off-by: Shakeel Butt shakeelb@google.com Reported-by: Jia He hejianet@gmail.com Tested-by: Jia He hejianet@gmail.com Acked-by: Michal Hocko mhocko@suse.com Acked-by: Hillf Danton hillf.zj@alibaba-inc.com Acked-by: Minchan Kim minchan@kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Cc: Dmitry Shmidt dimitrysh@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- include/linux/mmzone.h | 2 ++ mm/internal.h | 6 ++++++ mm/page_alloc.c | 9 ++------- mm/vmscan.c | 47 ++++++++++++++++++++++++++++++++--------------- mm/vmstat.c | 2 +- 5 files changed, 43 insertions(+), 23 deletions(-)

--- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -633,6 +633,8 @@ typedef struct pglist_data { int kswapd_order; enum zone_type kswapd_classzone_idx;

+ int kswapd_failures; /* Number of 'reclaimed == 0' runs */ + #ifdef CONFIG_COMPACTION int kcompactd_max_order; enum zone_type kcompactd_classzone_idx; --- a/mm/internal.h +++ b/mm/internal.h @@ -74,6 +74,12 @@ static inline void set_page_refcounted(s extern unsigned long highest_memmap_pfn;

/* + * Maximum number of reclaim retries without progress before the OOM + * killer is consider the only way forward. + */ +#define MAX_RECLAIM_RETRIES 16 + +/* * in mm/vmscan.c: */ extern int isolate_lru_page(struct page *page); --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3422,12 +3422,6 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_ma }

/* - * Maximum number of reclaim retries without any progress before OOM killer - * is consider as the only way to move forward. - */ -#define MAX_RECLAIM_RETRIES 16 - -/* * Checks whether it makes sense to retry the reclaim to make a forward progress * for the given allocation request. * The reclaim feedback represented by did_some_progress (any progress during @@ -4385,7 +4379,8 @@ void show_free_areas(unsigned int filter K(node_page_state(pgdat, NR_WRITEBACK_TEMP)), K(node_page_state(pgdat, NR_UNSTABLE_NFS)), node_page_state(pgdat, NR_PAGES_SCANNED), - !pgdat_reclaimable(pgdat) ? "yes" : "no"); + pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ? + "yes" : "no"); }

for_each_populated_zone(zone) { --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2606,6 +2606,15 @@ static bool shrink_node(pg_data_t *pgdat } while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed, sc->nr_scanned - nr_scanned, sc));

+ /* + * Kswapd gives up on balancing particular nodes after too + * many failures to reclaim anything from them and goes to + * sleep. On reclaim progress, reset the failure counter. A + * successful direct reclaim run will revive a dormant kswapd. + */ + if (reclaimable) + pgdat->kswapd_failures = 0; + return reclaimable; }

@@ -2680,10 +2689,6 @@ static void shrink_zones(struct zonelist GFP_KERNEL | __GFP_HARDWALL)) continue;

- if (sc->priority != DEF_PRIORITY && - !pgdat_reclaimable(zone->zone_pgdat)) - continue; /* Let kswapd poll it */ - /* * If we already have plenty of memory free for * compaction in this zone, don't free any more. @@ -2820,7 +2825,7 @@ retry: return 0; }

-static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) +static bool allow_direct_reclaim(pg_data_t *pgdat) { struct zone *zone; unsigned long pfmemalloc_reserve = 0; @@ -2828,6 +2833,9 @@ static bool pfmemalloc_watermark_ok(pg_d int i; bool wmark_ok;

+ if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES) + return true; + for (i = 0; i <= ZONE_NORMAL; i++) { zone = &pgdat->node_zones[i]; if (!managed_zone(zone) || @@ -2908,7 +2916,7 @@ static bool throttle_direct_reclaim(gfp_

/* Throttle based on the first usable node */ pgdat = zone->zone_pgdat; - if (pfmemalloc_watermark_ok(pgdat)) + if (allow_direct_reclaim(pgdat)) goto out; break; } @@ -2930,14 +2938,14 @@ static bool throttle_direct_reclaim(gfp_ */ if (!(gfp_mask & __GFP_FS)) { wait_event_interruptible_timeout(pgdat->pfmemalloc_wait, - pfmemalloc_watermark_ok(pgdat), HZ); + allow_direct_reclaim(pgdat), HZ);

goto check_pending; }

/* Throttle until kswapd wakes the process */ wait_event_killable(zone->zone_pgdat->pfmemalloc_wait, - pfmemalloc_watermark_ok(pgdat)); + allow_direct_reclaim(pgdat));

check_pending: if (fatal_signal_pending(current)) @@ -3116,7 +3124,7 @@ static bool prepare_kswapd_sleep(pg_data

/* * The throttled processes are normally woken up in balance_pgdat() as - * soon as pfmemalloc_watermark_ok() is true. But there is a potential + * soon as allow_direct_reclaim() is true. But there is a potential * race between when kswapd checks the watermarks and a process gets * throttled. There is also a potential race if processes get * throttled, kswapd wakes, a large process exits thereby balancing the @@ -3130,6 +3138,10 @@ static bool prepare_kswapd_sleep(pg_data if (waitqueue_active(&pgdat->pfmemalloc_wait)) wake_up_all(&pgdat->pfmemalloc_wait);

+ /* Hopeless node, leave it to direct reclaim */ + if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES) + return true; + for (i = 0; i <= classzone_idx; i++) { struct zone *zone = pgdat->node_zones + i;

@@ -3216,9 +3228,9 @@ static int balance_pgdat(pg_data_t *pgda count_vm_event(PAGEOUTRUN);

do { + unsigned long nr_reclaimed = sc.nr_reclaimed; bool raise_priority = true;

- sc.nr_reclaimed = 0; sc.reclaim_idx = classzone_idx;

/* @@ -3297,7 +3309,7 @@ static int balance_pgdat(pg_data_t *pgda * able to safely make forward progress. Wake them */ if (waitqueue_active(&pgdat->pfmemalloc_wait) && - pfmemalloc_watermark_ok(pgdat)) + allow_direct_reclaim(pgdat)) wake_up_all(&pgdat->pfmemalloc_wait);

/* Check if kswapd should be suspending */ @@ -3308,10 +3320,14 @@ static int balance_pgdat(pg_data_t *pgda * Raise priority if scanning rate is too low or there was no * progress in reclaiming pages */ - if (raise_priority || !sc.nr_reclaimed) + nr_reclaimed = sc.nr_reclaimed - nr_reclaimed; + if (raise_priority || !nr_reclaimed) sc.priority--; } while (sc.priority >= 1);

+ if (!sc.nr_reclaimed) + pgdat->kswapd_failures++; + out: /* * Return the order kswapd stopped reclaiming at as @@ -3511,6 +3527,10 @@ void wakeup_kswapd(struct zone *zone, in if (!waitqueue_active(&pgdat->kswapd_wait)) return;

+ /* Hopeless node, leave it to direct reclaim */ + if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES) + return; + /* Only wake kswapd if all zones are unbalanced */ for (z = 0; z <= classzone_idx; z++) { zone = pgdat->node_zones + z; @@ -3781,9 +3801,6 @@ int node_reclaim(struct pglist_data *pgd sum_zone_node_page_state(pgdat->node_id, NR_SLAB_RECLAIMABLE) <= pgdat->min_slab_pages) return NODE_RECLAIM_FULL;

- if (!pgdat_reclaimable(pgdat)) - return NODE_RECLAIM_FULL; - /* * Do not scan if the allocation should not be delayed. */ --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -1421,7 +1421,7 @@ static void zoneinfo_show_print(struct s "\n node_unreclaimable: %u" "\n start_pfn: %lu" "\n node_inactive_ratio: %u", - !pgdat_reclaimable(zone->zone_pgdat), + pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES, zone->zone_start_pfn, zone->zone_pgdat->inactive_ratio); seq_putc(m, '\n');

Patches currently in stable-queue which might be from hannes@cmpxchg.org are

queue-4.9/mm-fix-100-cpu-kswapd-busyloop-on-unreclaimable-nodes.patch queue-4.9/mm-page_alloc-fix-potential-false-positive-in-__zone_watermark_ok.patch