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

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

On Thu, 29 Jun 2023 23:00:07 -0700 (PDT)
Hugh Dickins <hughd@google.com> wrote:

> On Thu, 29 Jun 2023, Gerald Schaefer wrote:
> > On Thu, 29 Jun 2023 12:22:24 -0300
> > Jason Gunthorpe <jgg@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.  
> 
> I find myself half-agreeing with everyone.
> 
> I agree with Gerald that s390 should keep close to what it is already
> doing (except for adding pte_free_defer()): that changing its strategy
> and implementation to be much more like powerpc, is a job for some other
> occasion (and would depend on gathering data about how well each does).
> 
> But I agree with Jason that the powerpc solution we ended up with cut
> out a lot of unnecessary complication: it shifts the RCU delay from
> when pte_free_defer() is called, to when the shared page comes to be
> freed; which may be a lot later, and might not be welcome in a common
> path, but is quite okay for the uncommon pte_free_defer().

Ok, I guess I must admit that I completely ignored the latest progress in
the powerpc thread, and therefore was not up-to-date. Still had the older
approach in mind, where you also checked for pt_frag_refcount to avoid
double call_rcu().

The new approach sounds very reasonable, and I also like your latest
s390 patch from a first glance. Need to get more up-to-date with PageActive
and maybe also powerpc approach, and give this some proper review tomorrow.

> 
> And I agree with Alexander that pte_free_lower() and pte_free_upper()
> are better names than pte_free_now0() and pte_free_now1(): I was going
> to make that change, except all those functions disappear if we follow
> Jason's advice and switch the call_rcu() to when freeing the page.
> 
> (Lower and upper seem unambiguous to me: Gerald, does your confusion
> come just from the way they are shown the wrong way round in the PP AA
> diagram?  I corrected that in my patch, but you reverted it in yours.)

Ah yes, that could well be, and unfortunately I did not notice that you
fixed that in the comment. I only saw that you "fixed" the bit numbering
from 01234567 to 76543210, which I think is wrong on big-endian s390,
and therefore I simply removed that complete hunk.

But thanks a lot for pointing to that! We will certainly want to fix that
comment in a later patch, to reduce some or maybe all of the (at least
my) upper/lower confusion.