[PATCH] perf/core: fix mlock accounting in perf_mmap()

[Song Liu <songliubraving@fb.com>]

sysctl_perf_event_mlock and user->locked_vm can change value
independently, so we can't guarantee:

    user->locked_vm <= user_lock_limit

When user->locked_vm is larger than user_lock_limit, we cannot simply
update extra and user_extra as:

    extra = user_locked - user_lock_limit;
    user_extra -= extra;

Otherwise, user_extra will be negative. In extreme cases, this may lead to
negative user->locked_vm (until this perf-mmap is closed), which break
locked_vm badly.

Fix this with two separate conditions, which make sure user_extra is
always positive.

Fixes: c4b75479741c ("perf/core: Make the mlock accounting simple again")
Signed-off-by: Song Liu <songliubraving@fb.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 kernel/events/core.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a1f8bde19b56..89acdd1574ef 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5920,11 +5920,31 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 
 	if (user_locked > user_lock_limit) {
 		/*
-		 * charge locked_vm until it hits user_lock_limit;
-		 * charge the rest from pinned_vm
+		 * sysctl_perf_event_mlock and user->locked_vm can change
+		 * value independently, so we can't guarantee:
+		 *
+		 *    user->locked_vm <= user_lock_limit
+		 *
+		 * We need be careful to make sure user_extra >=0.
+		 *
+		 * Using "user_locked - user_extra" to avoid calling
+		 * atomic_long_read() again.
 		 */
-		extra = user_locked - user_lock_limit;
-		user_extra -= extra;
+		if (user_locked - user_extra >= user_lock_limit) {
+			/*
+			 * already used all user_locked_limit, charge all
+			 * to pinned_vm
+			 */
+			extra = user_extra;
+			user_extra = 0;
+		} else {
+			/*
+			 * charge locked_vm until it hits user_lock_limit;
+			 * charge the rest from pinned_vm
+			 */
+			extra = user_locked - user_lock_limit;
+			user_extra -= extra;
+		}
 	}
 
 	lock_limit = rlimit(RLIMIT_MEMLOCK);
-- 
2.17.1