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Date:   Thu, 4 Apr 2019 15:46:31 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Al Viro <viro@zeniv.linux.org.uk>
cc:     "Tobin C. Harding" <tobin@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Alexander Viro <viro@ftp.linux.org.uk>,
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        Pekka Enberg <penberg@cs.helsinki.fi>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Matthew Wilcox <willy@infradead.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Andreas Dilger <adilger@dilger.ca>,
        Waiman Long <longman@redhat.com>,
        Tycho Andersen <tycho@tycho.ws>, Theodore Ts'o <tytso@mit.edu>,
        Andi Kleen <ak@linux.intel.com>,
        David Chinner <david@fromorbit.com>,
        Nick Piggin <npiggin@gmail.com>,
        Rik van Riel <riel@redhat.com>,
        Hugh Dickins <hughd@google.com>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH v2 14/14] dcache: Implement object migration
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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.