Date:	Fri, 3 Oct 2014 17:44:46 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, Vladimir Davydov <vdavydov@...allels.com>,
	Greg Thelen <gthelen@...gle.com>, Dave Hansen <dave@...1.net>,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] mm: memcontrol: lockless page counters

On Thu 02-10-14 15:52:14, Johannes Weiner wrote:
> On Thu, Oct 02, 2014 at 11:01:35AM -0400, Johannes Weiner wrote:
> > On Tue, Sep 30, 2014 at 01:06:22PM +0200, Michal Hocko wrote:
> > > > +/**
> > > > + * page_counter_limit - limit the number of pages allowed
> > > > + * @counter: counter
> > > > + * @limit: limit to set
> > > > + *
> > > > + * Returns 0 on success, -EBUSY if the current number of pages on the
> > > > + * counter already exceeds the specified limit.
> > > > + *
> > > > + * The caller must serialize invocations on the same counter.
> > > > + */
> > > > +int page_counter_limit(struct page_counter *counter, unsigned long limit)
> > > > +{
> > > > +	for (;;) {
> > > > +		unsigned long old;
> > > > +		long count;
> > > > +
> > > > +		count = atomic_long_read(&counter->count);
> > > > +
> > > > +		old = xchg(&counter->limit, limit);
> > > > +
> > > > +		if (atomic_long_read(&counter->count) != count) {
> > > > +			counter->limit = old;
> > > > +			continue;
> > > > +		}
> > > > +
> > > > +		if (count > limit) {
> > > > +			counter->limit = old;
> > > > +			return -EBUSY;
> > > > +		}
> > > 
> > > Ordering doesn't make much sense to me here. Say you really want to set
> > > limit < count. You are effectively pushing all concurrent charges to
> > > the reclaim even though you would revert your change and return with
> > > EBUSY later on.
> > >
> > > Wouldn't (count > limit) check make more sense right after the first
> > > atomic_long_read?
> > > Also the second count check should be sufficient to check > count and
> > > retry only when the count has increased.
> > > Finally continuous flow of charges can keep this loop running for quite
> > > some time and trigger lockup detector. cond_resched before continue
> > > would handle that. Something like the following:
> > > 
> > > 	for (;;) {
> > > 		unsigned long old;
> > > 		long count;
> > > 
> > > 		count = atomic_long_read(&counter->count);
> > > 		if (count > limit)
> > > 			return -EBUSY;
> > > 
> > > 		old = xchg(&counter->limit, limit);
> > > 
> > > 		/* Recheck for concurrent charges */
> > > 		if (atomic_long_read(&counter->count) > count) {
> > > 			counter->limit = old;
> > > 			cond_resched();
> > > 			continue;
> > > 		}
> > > 
> > > 		return 0;
> > > 	}
> > 
> > This is susceptible to spurious -EBUSY during races with speculative
> > charges and uncharges.  My code avoids that by retrying until we set
> > the limit without any concurrent counter operations first, before
> > moving on to implementing policy and rollback.
> > 
> > Some reclaim activity caused by a limit that the user is trying to set
> > anyway should be okay.  I'd rather have a reliable syscall.
> > 
> > But the cond_resched() is a good idea, I'll add that, thanks.
> 
> Thinking more about it, my code doesn't really avoid that if the
> speculative charges persist over the two reads, it just widens the
> window a bit.  But your suggestion seems indeed more readable,
> although I'd invert the second branch.
> 
> How about this delta on top?
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index 4bdab1c7a057..7eb17135d4a4 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -19,8 +19,8 @@ int page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
>  
>  	new = atomic_long_sub_return(nr_pages, &counter->count);
>  
> -	if (WARN_ON_ONCE(new < 0))
> -		atomic_long_add(nr_pages, &counter->count);
> +	/* More uncharges than charges? */
> +	WARN_ON_ONCE(new < 0);
>  
>  	return new > 0;
>  }
> @@ -146,29 +146,29 @@ int page_counter_limit(struct page_counter *counter, unsigned long limit)
>  		unsigned long old;
>  		long count;
>  
> -		count = atomic_long_read(&counter->count);
>  		/*
> +		 * Update the limit while making sure that it's not
> +		 * below the (concurrently changing) counter value.
> +		 *
>  		 * The xchg implies two full memory barriers before
>  		 * and after, so the read-swap-read is ordered and
>  		 * ensures coherency with page_counter_try_charge():
>  		 * that function modifies the count before checking
>  		 * the limit, so if it sees the old limit, we see the
> -		 * modified counter and retry.  This guarantees we
> -		 * never successfully set a limit below the counter.
> +		 * modified counter and retry.
>  		 */
> -		old = xchg(&counter->limit, limit);
> -
> -		if (atomic_long_read(&counter->count) != count) {
> -			counter->limit = old;
> -			continue;
> -		}
> +		count = atomic_long_read(&counter->count);
>  
> -		if (count > limit) {
> -			counter->limit = old;
> +		if (count > limit)
>  			return -EBUSY;
> -		}
>  
> -		return 0;
> +		old = xchg(&counter->limit, limit);
> +
> +		if (atomic_long_read(&counter->count) <= count)
> +			return 0;
> +
> +		counter->limit = old;
> +		cond_resched();
>  	}
>  }

Looks good to me.

Thanks!
-- 
Michal Hocko
SUSE Labs
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