[Devel] [PATCH rh7 1/2] mm: fix division by zero in dcache_is_low()

[Andrey Ryabinin]

At first we check if sysctl_vfs_cache_min_ratio <= 0 and
if it's not we use it as denominator. If sysctl_vfs_cache_min_ratio
set to zero after the check but before division it would cause
division by zero, hence the kernel crash:

	divide error: 0000 [#1] SMP
	RIP: 0010:[<ffffffff81206fb5>]  [<ffffffff81206fb5>] mem_cgroup_dcache_is_low+0x55/0x80
	Call Trace:
	 super_cache_count+0xf4/0x180
	 shrink_slab+0x166/0x410
	 shrink_zone+0x11a/0x2d0
	 do_try_to_free_pages+0x1a0/0x570
	 try_to_free_mem_cgroup_pages+0xc6/0x160
	 mem_cgroup_reclaim+0x6b/0x180 [kpatch_cumulative_26_1_r1]
	 try_charge+0x18d/0x4e0 [kpatch_cumulative_26_1_r1]
	 mem_cgroup_try_charge+0x78/0x130
	 add_to_page_cache_locked+0x97/0x300
	 alloc_pages_current+0xaa/0x170
	 add_to_page_cache_lru+0x37/0xb0
	 grab_cache_page_write_begin+0x89/0xd0
	 ext4_da_write_begin+0xad/0x3a0 [ext4]
	 generic_file_buffered_write_iter+0x107/0x280
	 generic_file_write_iter+0x183/0x3c0
	 generic_file_aio_write+0x8b/0xb0
	 generic_file_aio_write+0x59/0xa0
	 ext4_file_write+0xdb/0x470 [ext4]
	 set_fd_set+0x21/0x30
	 core_sys_select+0x245/0x300
	 do_sync_write+0x90/0xe0
	 vfs_write+0xbd/0x1e0
	 SyS_write+0x7f/0xe0
	 system_call_fastpath+0x16/0x1b

Use READ_ONCE to cache sysctl_vfs_cache_min_ratio value into the
local variable, so we could use the cached value and not worry
about racy sysctl_vfs_cache_min_ratio updates.

https://jira.sw.ru/browse/PSBM-69018
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/super.c      | 5 +++--
 mm/memcontrol.c | 6 +++++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 7470621cbf4..9915a9bcd3a 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -51,8 +51,9 @@ static char *sb_writers_name[SB_FREEZE_LEVELS] = {
 static bool dcache_is_low(struct mem_cgroup *memcg)
 {
 	unsigned long anon, file, dcache;
+	int vfs_cache_min_ratio = READ_ONCE(sysctl_vfs_cache_min_ratio);
 
-	if (sysctl_vfs_cache_min_ratio <= 0)
+	if (vfs_cache_min_ratio <= 0)
 		return false;
 
 	if (memcg)
@@ -62,7 +63,7 @@ static bool dcache_is_low(struct mem_cgroup *memcg)
 	file = global_page_state(NR_FILE_PAGES);
 	dcache = global_page_state(NR_SLAB_RECLAIMABLE);
 
-	return dcache / sysctl_vfs_cache_min_ratio <
+	return dcache / vfs_cache_min_ratio <
 			(anon + file + dcache) / 100;
 }
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b1bc0924f6d..9678957bf22 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1588,12 +1588,16 @@ int mem_cgroup_inactive_anon_is_low(struct lruvec *lruvec)
 bool mem_cgroup_dcache_is_low(struct mem_cgroup *memcg)
 {
 	unsigned long anon, file, dcache;
+	int vfs_cache_min_ratio = READ_ONCE(sysctl_vfs_cache_min_ratio);
+
+	if (vfs_cache_min_ratio <= 0)
+		return false;
 
 	anon = mem_cgroup_read_stat(memcg, MEM_CGROUP_STAT_RSS);
 	file = mem_cgroup_read_stat(memcg, MEM_CGROUP_STAT_CACHE);
 	dcache = mem_cgroup_read_stat(memcg, MEM_CGROUP_STAT_SLAB_RECLAIMABLE);
 
-	return dcache / sysctl_vfs_cache_min_ratio <
+	return dcache / vfs_cache_min_ratio <
 			(anon + file + dcache) / 100;
 }
 
-- 
2.13.0