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

[Miklos Szeredi]

On Wed, Apr 3, 2019 at 9:05 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Apr 03, 2019 at 07:24:54PM +0100, Al Viro wrote:
>
> > 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.
> >
> > If you mean "kick out everything in this page with zero refcount" - that
> > can be done (see further in the thread).
> >
> > 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.
>
> FWIW, some theory: the only kind of long-term reference that can
> be killed off by memory pressure is that from child to parent.
> Anything else (e.g. an opened file, current directory, mountpoint,
> etc.) is out of limits - it either won't be going away until
> the thing is not pinned anymore (close, chdir, etc.) *or*
> it really shouldn't be ("VM wants this mountpoint dentry freed,
> so just dissolve the mount" is a bloody bad idea for obvious
> reasons).

Well, theoretically we could do two levels of references, where the
long term reference is stable and contains an rcu protected unstable
reference to the real object.   In the likely case when only read-only
access to the object is needed (d_lookup) then the cost is an extra
dereference and the associated additional cache usage.  If read-write
access is needed to object, then extra locking is needed to protect
against concurrent migration.  So there's non-trivial cost in addition
to the added complexity, and I don't see it actually making sense in
practice.   But maybe someone can expand this idea to something
practicable...

Thanks,
Miklos