linux-kernel - Re: Will the recent memory leak fixes be backported to longterm kernels?

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Message-ID: <20181102193827.GA18024@castle.DHCP.thefacebook.com>
Date:   Fri, 2 Nov 2018 19:38:35 +0000
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Dexuan Cui <decui@...rosoft.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Rik van Riel <riel@...riel.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Stable@...r.kernel.org" <Stable@...r.kernel.org>
Subject: Re: Will the recent memory leak fixes be backported to longterm
 kernels?

On Fri, Nov 02, 2018 at 06:48:23PM +0100, Michal Hocko wrote:
> On Fri 02-11-18 17:25:58, Roman Gushchin wrote:
> > On Fri, Nov 02, 2018 at 05:51:47PM +0100, Michal Hocko wrote:
> > > On Fri 02-11-18 16:22:41, Roman Gushchin wrote:
> [...]
> > > > 2) We do forget to scan the last page in the LRU list. So if we ended up with
> > > > 1-page long LRU, it can stay there basically forever.
> > > 
> > > Why 
> > > 		/*
> > > 		 * If the cgroup's already been deleted, make sure to
> > > 		 * scrape out the remaining cache.
> > > 		 */
> > > 		if (!scan && !mem_cgroup_online(memcg))
> > > 			scan = min(size, SWAP_CLUSTER_MAX);
> > > 
> > > in get_scan_count doesn't work for that case?
> > 
> > No, it doesn't. Let's look at the whole picture:
> > 
> > 		size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
> > 		scan = size >> sc->priority;
> > 		/*
> > 		 * If the cgroup's already been deleted, make sure to
> > 		 * scrape out the remaining cache.
> > 		 */
> > 		if (!scan && !mem_cgroup_online(memcg))
> > 			scan = min(size, SWAP_CLUSTER_MAX);
> > 
> > If size == 1, scan == 0 => scan = min(1, 32) == 1.
> > And after proportional adjustment we'll have 0.
> 
> My friday brain hurst when looking at this but if it doesn't work as
> advertized then it should be fixed. I do not see any of your patches to
> touch this logic so how come it would work after them applied?

This part works as expected. But the following
	scan = div64_u64(scan * fraction[file], denominator);
reliable turns 1 page to scan to 0 pages to scan.

And this is the issue which my patches do address.

Thanks!