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

[Andrew Morton]

On Thu, 25 Oct 2018 11:23:52 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> 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.

Yup.  Sasha, can you please take care of this?