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Date:   Wed, 24 Oct 2018 15:19:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Michal Hocko <mhocko@kernel.org>,
        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: <20181024151950.36fe2c41957d807756f587ca@linux-foundation.org>
In-Reply-To: <20181023164302.20436-1-guro@fb.com>
References: <20181023164302.20436-1-guro@fb.com>
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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?