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SubjectRe: [PATCH] perf/core: fix mlock accounting in perf_mmap()
Date
Song Liu <songliubraving@fb.com> writes:

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

This means: if the sysctl got sufficiently decreased, so that the
existing locked_vm exceeds it, we need to deal with the overflow, right?

> 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;
> + }

How about the below for the sake of brevity?

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 763cf34b5a63..632505ce6c12 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5917,7 +5917,14 @@ 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);
+ /*
+ * If perf_event_mlock has changed since earlier mmaps, so that
+ * it's smaller than user->locked_vm, discard the overflow.
+ */
+ if (user_locked > user_lock_limit)
+ user_locked = user_lock_limit;
+ user_locked += user_extra;

if (user_locked > user_lock_limit) {
/*
Regards,
--
Alex

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