[PATCH] mm/util: fix a data race in __vm_enough_memory()

[Qian Cai]

"vm_committed_as.count" could be accessed concurrently as reported by
KCSAN,

 read to 0xffffffff923164f8 of 8 bytes by task 1268 on cpu 38:
  __vm_enough_memory+0x43/0x280 mm/util.c:801
  mmap_region+0x1b2/0xb90 mm/mmap.c:1726
  do_mmap+0x45c/0x700
  vm_mmap_pgoff+0xc0/0x130
  vm_mmap+0x71/0x90
  elf_map+0xa1/0x1b0
  load_elf_binary+0x9de/0x2180
  search_binary_handler+0xd8/0x2b0
  __do_execve_file+0xb61/0x1080
  __x64_sys_execve+0x5f/0x70
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

 write to 0xffffffff923164f8 of 8 bytes by task 1265 on cpu 41:
  percpu_counter_add_batch+0x83/0xd0 lib/percpu_counter.c:91
  exit_mmap+0x178/0x220 include/linux/mman.h:68
  mmput+0x10e/0x270
  flush_old_exec+0x572/0xfe0
  load_elf_binary+0x467/0x2180
  search_binary_handler+0xd8/0x2b0
  __do_execve_file+0xb61/0x1080
  __x64_sys_execve+0x5f/0x70
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Since only the read is operating as lockless, fix it by using
READ_ONLY() for it to avoid any possible false warning due to load
tearing.

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/util.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 988d11e6c17c..58cd8f28651c 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -798,7 +798,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 {
 	long allowed;
 
-	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
+	VM_WARN_ONCE(READ_ONCE(vm_committed_as.count) <
 			-(s64)vm_committed_as_batch * num_online_cpus(),
 			"memory commitment underflow");
 
-- 
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