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Subject[PATCH v2] perf/core: fix mlock accounting in perf_mmap()
Date
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 by adjusting user_locked before calculating extra and user_extra.

Fixes: c4b75479741c ("perf/core: Make the mlock accounting simple again")
Signed-off-by: Song Liu <songliubraving@fb.com>
Suggested-by: Alexander Shishkin <alexander.shishkin@linux.intel.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 | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2173c23c25b4..d25f2de45996 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5916,8 +5916,19 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
*/
user_lock_limit *= num_online_cpus();

- user_locked = atomic_long_read(&user->locked_vm) + user_extra;
+ user_locked = atomic_long_read(&user->locked_vm);

+ /*
+ * sysctl_perf_event_mlock and user->locked_vm can change value
+ * independently. so we can't guarantee:
+ * user->locked_vm <= user_lock_limit
+ *
+ * Adjust user_locked to be <= user_lock_limit so we can calcualte
+ * correct extra and user_extra.
+ */
+ user_locked = min_t(unsigned long, user_locked, user_lock_limit);
+
+ user_locked += user_extra;
if (user_locked > user_lock_limit) {
/*
* charge locked_vm until it hits user_lock_limit;
--
2.17.1
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