Re: [RFC PATCH v2 14/14] dcache: Implement object migration

[Christopher Lameter]

On Wed, 3 Apr 2019, Al Viro wrote:

> > This is an RFC and we want to know how to do this right.
>
> If by "how to do it right" you mean "expedit kicking out something with
> non-zero refcount" - there's no way to do that.  Nothing even remotely
> sane.

Sure we know that.

> If you mean "kick out everything in this page with zero refcount" - that
> can be done (see further in the thread).

Ok that would already be progress. If we can use this to liberate some
slab pages with just a few dentry object then it may be worthwhile.

> Look, dentries and inodes are really, really not relocatable.  If they
> can be evicted by memory pressure - sure, we can do that for a given
> set (e.g. "everything in that page").  But that's it - if memory
> pressure would _not_ get rid of that one, there's nothing to be done.
> Again, all VM can do is to simulate shrinker hitting hard on given
> bunch (rather than buggering the entire cache).  If filesystem (or
> something in VFS) says "it's busy", it bloody well _is_ busy and
> won't be going away until it ceases to be such.

Right. Thats why the patch attempted to check for these things to avoid
touching such objects.