linux-kernel - Re: [PATCH] perf/core: fix mlock accounting in perf

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Date:   Mon, 20 Jan 2020 10:24:00 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Song Liu <songliubraving@...com>, linux-kernel@...r.kernel.org
Cc:     kernel-team@...com, Song Liu <songliubraving@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH] perf/core: fix mlock accounting in perf_mmap()

Song Liu <songliubraving@...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