linux-kernel - Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU

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Date:   Tue, 11 Feb 2020 14:05:38 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Dave Chinner <david@...morbit.com>,
        Yafang Shao <laoar.shao@...il.com>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, kernel-team@...com
Subject: Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker
 LRU

On Tue, 2020-02-11 at 12:55 -0500, Johannes Weiner wrote:
> The VFS inode shrinker is currently allowed to reclaim inodes with
> populated page cache. As a result it can drop gigabytes of hot and
> active page cache on the floor without consulting the VM (recorded as
> "inodesteal" events in /proc/vmstat).
> 
> This causes real problems in practice. Consider for example how the
> VM
> would cache a source tree, such as the Linux git tree. As large parts
> of the checked out files and the object database are accessed
> repeatedly, the page cache holding this data gets moved to the active
> list, where it's fully (and indefinitely) insulated from one-off
> cache
> moving through the inactive list.

> This behavior of invalidating page cache from the inode shrinker goes
> back to even before the git import of the kernel tree. It may have
> been less noticeable when the VM itself didn't have real workingset
> protection, and floods of one-off cache would push out any active
> cache over time anyway. But the VM has come a long way since then and
> the inode shrinker is now actively subverting its caching strategy.

Two things come to mind when looking at this:
- highmem
- NUMA

IIRC one of the reasons reclaim is done in this way is
because a page cache page in one area of memory (highmem,
or a NUMA node) can end up pinning inode slab memory in
another memory area (normal zone, other NUMA node).

I do not know how much of a concern that still is nowadays,
but it seemed something worth bringing up.

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