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

[Rik van Riel <riel@surriel.com>]

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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.

-- 
All Rights Reversed.

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