Re: [PATCH v2] mm/swap: piggyback lru_add_drain_all() calls

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 04 Oct 2019 16:09:22 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:

> This is very slow operation. There is no reason to do it again if somebody
> else already drained all per-cpu vectors while we waited for lock.
> 
> Piggyback on drain started and finished while we waited for lock:
> all pages pended at the time of our enter were drained from vectors.
> 
> Callers like POSIX_FADV_DONTNEED retry their operations once after
> draining per-cpu vectors when pages have unexpected references.
> 
> ...
>
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -708,9 +708,10 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
>   */
>  void lru_add_drain_all(void)
>  {
> +	static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
>  	static DEFINE_MUTEX(lock);
>  	static struct cpumask has_work;
> -	int cpu;
> +	int cpu, seq;
>  
>  	/*
>  	 * Make sure nobody triggers this path before mm_percpu_wq is fully
> @@ -719,7 +720,19 @@ void lru_add_drain_all(void)
>  	if (WARN_ON(!mm_percpu_wq))
>  		return;
>  
> +	seq = raw_read_seqcount_latch(&seqcount);
> +
>  	mutex_lock(&lock);
> +
> +	/*
> +	 * Piggyback on drain started and finished while we waited for lock:
> +	 * all pages pended at the time of our enter were drained from vectors.
> +	 */
> +	if (__read_seqcount_retry(&seqcount, seq))
> +		goto done;
> +
> +	raw_write_seqcount_latch(&seqcount);
> +
>  	cpumask_clear(&has_work);
>  
>  	for_each_online_cpu(cpu) {
> @@ -740,6 +753,7 @@ void lru_add_drain_all(void)
>  	for_each_cpu(cpu, &has_work)
>  		flush_work(&per_cpu(lru_add_drain_work, cpu));
>  
> +done:
>  	mutex_unlock(&lock);
>  }

I'm not sure this works as intended.

Suppose CPU #30 is presently executing the for_each_online_cpu() loop
and has reached CPU #15's per-cpu data.

Now CPU #2 comes along, adds some pages to its per-cpu vectors then
calls lru_add_drain_all().  AFAICT the code will assume that CPU #30
has flushed out all of the pages which CPU #2 just added, but that
isn't the case.

Moving the raw_write_seqcount_latch() to the point where all processing
has completed might fix?