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

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Apr 03, 2019 at 05:56:27PM +0000, Christopher Lameter wrote:
> On Wed, 3 Apr 2019, Al Viro wrote:
> 
> > Let's do d_invalidate() on random dentries and hope they go away.
> > With convoluted and brittle logics for deciding which ones to
> > spare, which is actually wrong.  This will pick mountpoints
> > and tear them out, to start with.
> >
> > NAKed-by: Al Viro <viro@zeniv.linux.org.uk>
> >
> > And this is a NAK for the entire approach; if it has a positive refcount,
> > LEAVE IT ALONE.  Period.  Don't play this kind of games, they are wrong.
> > d_invalidate() is not something that can be done to an arbitrary dentry.
> 
> Well could you help us figure out how to do it the right way? We (the MM
> guys) are having a hard time not being familiar with the filesystem stuff.
> 
> 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.

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.