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Date:   Thu, 25 Oct 2018 11:23:52 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Roman Gushchin <guro@fb.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>,
        Rik van Riel <riel@surriel.com>,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [RFC PATCH] mm: don't reclaim inodes with many attached pages
Message-ID: <20181025092352.GP18839@dhcp22.suse.cz>
References: <20181023164302.20436-1-guro@fb.com>
 <20181024151950.36fe2c41957d807756f587ca@linux-foundation.org>
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On Wed 24-10-18 15:19:50, Andrew Morton wrote:
> On Tue, 23 Oct 2018 16:43:29 +0000 Roman Gushchin <guro@fb.com> wrote:
> 
> > Spock reported that the commit 172b06c32b94 ("mm: slowly shrink slabs
> > with a relatively small number of objects") leads to a regression on
> > his setup: periodically the majority of the pagecache is evicted
> > without an obvious reason, while before the change the amount of free
> > memory was balancing around the watermark.
> > 
> > The reason behind is that the mentioned above change created some
> > minimal background pressure on the inode cache. The problem is that
> > if an inode is considered to be reclaimed, all belonging pagecache
> > page are stripped, no matter how many of them are there. So, if a huge
> > multi-gigabyte file is cached in the memory, and the goal is to
> > reclaim only few slab objects (unused inodes), we still can eventually
> > evict all gigabytes of the pagecache at once.
> > 
> > The workload described by Spock has few large non-mapped files in the
> > pagecache, so it's especially noticeable.
> > 
> > To solve the problem let's postpone the reclaim of inodes, which have
> > more than 1 attached page. Let's wait until the pagecache pages will
> > be evicted naturally by scanning the corresponding LRU lists, and only
> > then reclaim the inode structure.
> 
> Is this regression serious enough to warrant fixing 4.19.1?

Let's not forget about stable tree(s) which backported 172b06c32b94. I
would suggest reverting there.
-- 
Michal Hocko
SUSE Labs