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

From: Roman Gushchin
Date: Fri Oct 26 2018 - 11:57:25 EST

On Fri, Oct 26, 2018 at 10:57:35AM +0200, Michal Hocko wrote:
> Spock doesn't seem to be cced here - fixed now
> On Tue 23-10-18 16:43:29, Roman Gushchin 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.
> Has this actually fixed/worked around the issue?

Spock wrote this earlier to me directly. I believe I can quote it here:

"Patch applied, looks good so far. System behaves like it was with
pre-4.18.15 kernels.
Also tried to add some user-level tests to the geneic background activity, like
- stat'ing a bunch of files
- streamed read several large files at once on ext4 and XFS
- random reads on the whole collection with a read size of 16K

I will be monitoring while fragmentation stacks up and report back if
something bad happens."

Spock, please let me know if you have any new results.