[PATCH v2 07/12] s390: add pte_free_defer() for pgtables sharing page

[Gerald Schaefer]

On Thu, 29 Jun 2023 12:22:24 -0300
Jason Gunthorpe <jgg at ziepe.ca> wrote:

> On Wed, Jun 28, 2023 at 10:08:08PM -0700, Hugh Dickins wrote:
> > On Wed, 28 Jun 2023, Gerald Schaefer wrote:  
> > > 
> > > As discussed in the other thread, we would rather go with less complexity,
> > > possibly switching to an approach w/o the list and fragment re-use in the
> > > future. For now, as a first step in that direction, we can try with not
> > > adding fragments back only for pte_free_defer(). Here is an adjusted
> > > version of your patch, copying most of your pte_free_defer() logic and
> > > also description, tested with LTP and all three of your patch series applied:  
> > 
> > Thanks, Gerald: I don't mind abandoning my 13/12 SLAB_TYPESAFE_BY_RCU
> > patch (posted with fewer Cc's to the s390 list last week), and switching
> > to your simpler who-cares-if-we-sometimes-don't-make-maximal-use-of-page
> > patch.
> > 
> > But I didn't get deep enough into it today to confirm it - and disappointed
> > that you've found it necessary to play with pt_frag_refcount in addition to
> > _refcount and HH bits.  No real problem with that, but my instinct says it
> > should be simpler.  

Yes, I also found it a bit awkward, but it seemed "good and simple enough",
to have something to go forward with, while my instinct was in line with yours.

> 
> Is there any reason it should be any different at all from what PPC is
> doing?
> 
> I still think the right thing to do here is make the PPC code common
> (with Hugh's proposed RCU modification) and just use it in both
> arches....

With the current approach, we would not add back fragments _only_ for
the new pte_free_defer() path, while keeping our cleverness for the other
paths. Not having a good overview of the negative impact wrt potential
memory waste, I would rather take small steps, if possible.

If we later switch to never adding back fragments, of course we should
try to be in line with PPC implementation.