Re: [RFC PATCH] mm: don't reclaim inodes with many attached pages

From: Michal Hocko
Date: Thu Oct 25 2018 - 05:23:57 EST

On Wed 24-10-18 15:19:50, Andrew Morton wrote:
> On Tue, 23 Oct 2018 16:43:29 +0000 Roman Gushchin <guro@xxxxxx> 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