mm/vmscan.c: fix data races using kswapd_classzone_idx

pgdat->kswapd_classzone_idx could be accessed concurrently in
wakeup_kswapd().  Plain writes and reads without any lock protection
result in data races.  Fix them by adding a pair of READ|WRITE_ONCE() as
well as saving a branch (compilers might well optimize the original code
in an unintentional way anyway).  While at it, also take care of
pgdat->kswapd_order and non-kswapd threads in allow_direct_reclaim().  The
data races were reported by KCSAN,

 BUG: KCSAN: data-race in wakeup_kswapd / wakeup_kswapd

 write to 0xffff9f427ffff2dc of 4 bytes by task 7454 on cpu 13:
  wakeup_kswapd+0xf1/0x400
  wakeup_kswapd at mm/vmscan.c:3967
  wake_all_kswapds+0x59/0xc0
  wake_all_kswapds at mm/page_alloc.c:4241
  __alloc_pages_slowpath+0xdcc/0x1290
  __alloc_pages_slowpath at mm/page_alloc.c:4512
  __alloc_pages_nodemask+0x3bb/0x450
  alloc_pages_vma+0x8a/0x2c0
  do_anonymous_page+0x16e/0x6f0
  __handle_mm_fault+0xcd5/0xd40
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

 1 lock held by mtest01/7454:
  #0: ffff9f425afe8808 (&mm->mmap_sem#2){++++}, at:
 do_page_fault+0x143/0x6f9
 do_user_addr_fault at arch/x86/mm/fault.c:1405
 (inlined by) do_page_fault at arch/x86/mm/fault.c:1539
 irq event stamp: 6944085
 count_memcg_event_mm+0x1a6/0x270
 count_memcg_event_mm+0x119/0x270
 __do_softirq+0x34c/0x57c
 irq_exit+0xa2/0xc0

 read to 0xffff9f427ffff2dc of 4 bytes by task 7472 on cpu 38:
  wakeup_kswapd+0xc8/0x400
  wake_all_kswapds+0x59/0xc0
  __alloc_pages_slowpath+0xdcc/0x1290
  __alloc_pages_nodemask+0x3bb/0x450
  alloc_pages_vma+0x8a/0x2c0
  do_anonymous_page+0x16e/0x6f0
  __handle_mm_fault+0xcd5/0xd40
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

 1 lock held by mtest01/7472:
  #0: ffff9f425a9ac148 (&mm->mmap_sem#2){++++}, at:
 do_page_fault+0x143/0x6f9
 irq event stamp: 6793561
 count_memcg_event_mm+0x1a6/0x270
 count_memcg_event_mm+0x119/0x270
 __do_softirq+0x34c/0x57c
 irq_exit+0xa2/0xc0

 BUG: KCSAN: data-race in kswapd / wakeup_kswapd

 write to 0xffff90973ffff2dc of 4 bytes by task 820 on cpu 6:
  kswapd+0x27c/0x8d0
  kthread+0x1e0/0x200
  ret_from_fork+0x27/0x50

 read to 0xffff90973ffff2dc of 4 bytes by task 6299 on cpu 0:
  wakeup_kswapd+0xf3/0x450
  wake_all_kswapds+0x59/0xc0
  __alloc_pages_slowpath+0xdcc/0x1290
  __alloc_pages_nodemask+0x3bb/0x450
  alloc_pages_vma+0x8a/0x2c0
  do_anonymous_page+0x170/0x700
  __handle_mm_fault+0xc9f/0xd00
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Marco Elver <elver@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Link: http://lkml.kernel.org/r/1582749472-5171-1-git-send-email-cai@lca.pw
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index af0b6f0b59cc51472b541a4babf8402ee4aea218..0eae0aa9df12e30f94b5eb918d56e49c9ab9929d 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3136,8 +3136,9 @@ static bool allow_direct_reclaim(pg_data_t *pgdat)
 
        /* kswapd must be awake if processes are being throttled */
        if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
-               pgdat->kswapd_classzone_idx = min(pgdat->kswapd_classzone_idx,
-                                               (enum zone_type)ZONE_NORMAL);
+               if (READ_ONCE(pgdat->kswapd_classzone_idx) > ZONE_NORMAL)
+                       WRITE_ONCE(pgdat->kswapd_classzone_idx, ZONE_NORMAL);
+
                wake_up_interruptible(&pgdat->kswapd_wait);
        }
 
@@ -3769,9 +3770,9 @@ out:
 static enum zone_type kswapd_classzone_idx(pg_data_t *pgdat,
                                           enum zone_type prev_classzone_idx)
 {
-       if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
-               return prev_classzone_idx;
-       return pgdat->kswapd_classzone_idx;
+       enum zone_type curr_idx = READ_ONCE(pgdat->kswapd_classzone_idx);
+
+       return curr_idx == MAX_NR_ZONES ? prev_classzone_idx : curr_idx;
 }
 
 static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
@@ -3815,8 +3816,11 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_o
                 * the previous request that slept prematurely.
                 */
                if (remaining) {
-                       pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
-                       pgdat->kswapd_order = max(pgdat->kswapd_order, reclaim_order);
+                       WRITE_ONCE(pgdat->kswapd_classzone_idx,
+                                  kswapd_classzone_idx(pgdat, classzone_idx));
+
+                       if (READ_ONCE(pgdat->kswapd_order) < reclaim_order)
+                               WRITE_ONCE(pgdat->kswapd_order, reclaim_order);
                }
 
                finish_wait(&pgdat->kswapd_wait, &wait);
@@ -3893,12 +3897,12 @@ static int kswapd(void *p)
        tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
        set_freezable();
 
-       pgdat->kswapd_order = 0;
-       pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
+       WRITE_ONCE(pgdat->kswapd_order, 0);
+       WRITE_ONCE(pgdat->kswapd_classzone_idx, MAX_NR_ZONES);
        for ( ; ; ) {
                bool ret;
 
-               alloc_order = reclaim_order = pgdat->kswapd_order;
+               alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
                classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
 
 kswapd_try_sleep:
@@ -3906,10 +3910,10 @@ kswapd_try_sleep:
                                        classzone_idx);
 
                /* Read the new order and classzone_idx */
-               alloc_order = reclaim_order = pgdat->kswapd_order;
+               alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
                classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
-               pgdat->kswapd_order = 0;
-               pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
+               WRITE_ONCE(pgdat->kswapd_order, 0);
+               WRITE_ONCE(pgdat->kswapd_classzone_idx, MAX_NR_ZONES);
 
                ret = try_to_freeze();
                if (kthread_should_stop())
@@ -3953,20 +3957,23 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
                   enum zone_type classzone_idx)
 {
        pg_data_t *pgdat;
+       enum zone_type curr_idx;
 
        if (!managed_zone(zone))
                return;
 
        if (!cpuset_zone_allowed(zone, gfp_flags))
                return;
+
        pgdat = zone->zone_pgdat;
+       curr_idx = READ_ONCE(pgdat->kswapd_classzone_idx);
+
+       if (curr_idx == MAX_NR_ZONES || curr_idx < classzone_idx)
+               WRITE_ONCE(pgdat->kswapd_classzone_idx, classzone_idx);
+
+       if (READ_ONCE(pgdat->kswapd_order) < order)
+               WRITE_ONCE(pgdat->kswapd_order, order);
 
-       if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
-               pgdat->kswapd_classzone_idx = classzone_idx;
-       else
-               pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
-                                                 classzone_idx);
-       pgdat->kswapd_order = max(pgdat->kswapd_order, order);
        if (!waitqueue_active(&pgdat->kswapd_wait))
                return;