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Date:   Mon, 5 Nov 2018 10:21:23 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Dexuan Cui <decui@microsoft.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Rik van Riel <riel@surriel.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Stable@vger.kernel.org" <Stable@vger.kernel.org>
Subject: Re: Will the recent memory leak fixes be backported to longterm
 kernels?
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On Fri 02-11-18 19:38:35, Roman Gushchin wrote:
> 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.

OK, 68600f623d69 ("mm: don't miss the last page because of round-off
error") sounds like a good and safe stable backport material.

-- 
Michal Hocko
SUSE Labs