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

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

On Wed, 5 Jul 2023 17:52:40 -0700 (PDT)
Hugh Dickins <hughd@google.com> wrote:

> On Wed, 5 Jul 2023, Alexander Gordeev wrote:
> > On Sat, Jul 01, 2023 at 09:32:38PM -0700, Hugh Dickins wrote:  
> > > On Thu, 29 Jun 2023, Hugh Dickins wrote:  
> > 
> > Hi Hugh,
> > 
> > ...
> >   
> > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> > > +{
> > > +	struct page *page;  
> > 
> > If I got your and Claudio conversation right, you were going to add
> > here WARN_ON_ONCE() in case of mm_alloc_pgste(mm)?  

Good point, thanks Alexander for noticing!

> 
> Well, Claudio approved, so I would have put it in, if we had stuck with
> that version which had "if (mm_alloc_pgste(mm)) {" in pte_free_defer();
> but once that went away, it became somewhat irrelevant... to me anyway.
> 
> But I don't mind adding it here, in the v3 I'll post when -rc1 is out,
> if it might help you guys - there is some point, since pte_free_defer()
> is a route which can usefully check for such a case, without confusion
> from harmless traffic from immediate frees of just-in-case allocations.
> 
> But don't expect it to catch all such cases (if they exist): another
> category of s390 page_table_free()s comes from the PageAnon
> zap_deposited_table() in zap_huge_pmd(): those tables might or might
> not have been exposed to userspace at some time in the past.

Right, after THP collapse, the previously active PTE table would be
deposited in this case, and then later freed in zap_deposited_table().
I guess we need to be very careful, if THP was ever enabled for KVM
guests.

> 
> I'll add the WARN_ON_ONCE in pte_free_defer() (after checking that
> WARN_ON_ONCE is the one we want - I get confused by all the different
> flavours of WARN, and have to check the header file each time to be
> sure of the syntax and semantics): but be aware that it won't be
> checking all potential cases.

Thanks, looks good.