[PATCH -next] mm/page_counter: mark intentional data races

[Qian Cai]

The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
had memcg->memsw->failcnt and ->watermark could be accessed concurrently
as reported by KCSAN,

 Reported by Kernel Concurrency Sanitizer on:
 BUG: KCSAN: data-race in page_counter_try_charge / page_counter_try_charge

 read to 0xffff8fb18c4cd190 of 8 bytes by task 1081 on cpu 59:
  page_counter_try_charge+0x4d/0x150 mm/page_counter.c:138
  try_charge+0x131/0xd50
  __memcg_kmem_charge_memcg+0x58/0x140
  __memcg_kmem_charge+0xcc/0x280
  __alloc_pages_nodemask+0x1e1/0x450
  alloc_pages_current+0xa6/0x120
  pte_alloc_one+0x17/0xd0
  __pte_alloc+0x3a/0x1f0
  copy_p4d_range+0xc36/0x1990
  copy_page_range+0x21d/0x360
  dup_mmap+0x5f5/0x7a0
  dup_mm+0xa2/0x240
  copy_process+0x1b3f/0x3460
  _do_fork+0xaa/0xa20
  __x64_sys_clone+0x13b/0x170
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

 write to 0xffff8fb18c4cd190 of 8 bytes by task 1153 on cpu 120:
  page_counter_try_charge+0x5b/0x150 mm/page_counter.c:139
  try_charge+0x131/0xd50
  mem_cgroup_try_charge+0x159/0x460
  mem_cgroup_try_charge_delay+0x3d/0xa0
  wp_page_copy+0x14d/0x930
  do_wp_page+0x107/0x7b0
  __handle_mm_fault+0xce6/0xd40
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

Since the failcnt and watermark are tolerant of some inaccuracy, a data
race will not be harmful, thus mark them as intentional data races with
the data_race() macro.

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/page_counter.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index de31470655f6..13934636eafd 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -82,8 +82,8 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
 		 * This is indeed racy, but we can live with some
 		 * inaccuracy in the watermark.
 		 */
-		if (new > c->watermark)
-			c->watermark = new;
+		if (data_race(new > c->watermark))
+			data_race(c->watermark = new);
 	}
 }
 
@@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter,
 			 * This is racy, but we can live with some
 			 * inaccuracy in the failcnt.
 			 */
-			c->failcnt++;
+			data_race(c->failcnt++);
 			*fail = c;
 			goto failed;
 		}
@@ -135,8 +135,8 @@ bool page_counter_try_charge(struct page_counter *counter,
 		 * Just like with failcnt, we can live with some
 		 * inaccuracy in the watermark.
 		 */
-		if (new > c->watermark)
-			c->watermark = new;
+		if (data_race(new > c->watermark))
+			data_race(c->watermark = new);
 	}
 	return true;
 
-- 
2.21.0 (Apple Git-122.2)