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

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Message-ID: <20190130055834.GC2107@castle.DHCP.thefacebook.com>
Date:   Wed, 30 Jan 2019 05:58:42 +0000
From:   Roman Gushchin <guro@...com>
To:     Sasha Levin <sashal@...nel.org>
CC:     Greg KH <greg@...ah.com>, Michal Hocko <mhocko@...nel.org>,
        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 Tue, Jan 29, 2019 at 07:23:56PM -0500, Sasha Levin wrote:
> On Fri, Dec 28, 2018 at 11:50:08AM +0100, Greg KH wrote:
> > On Mon, Nov 05, 2018 at 10:21:23AM +0100, Michal Hocko wrote:
> > > 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.
> > 
> > Thanks for this, now queued up.
> > 
> > greg k-h
> 
> It seems that 172b06c32b949 ("mm: slowly shrink slabs with a relatively
> small number of objects") and a76cf1a474d ("mm: don't reclaim inodes
> with many attached pages") cause a regression reported against the 4.19
> stable tree: https://bugzilla.kernel.org/show_bug.cgi?id=202441 .
> 
> Given the history and complexity of these (and other patches from that
> series) it would be nice to understand if this is something that will be
> fixed soon or should we look into reverting the series for now?

In that thread I've just suggested to give a chance to Rik's patch, which
hopefully will mitigate or easy the regression (
https://lkml.org/lkml/2019/1/28/1865 ).

Of course, we can simple revert those changes, but this will re-introduce
the memory leak, so I'd leave it as a last option.

Thanks!