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

[Hugh Dickins <hughd@google.com>]

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)?

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.

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.

Hugh

> 
> > +	page = virt_to_page(pgtable);
> > +	SetPageActive(page);
> > +	page_table_free(mm, (unsigned long *)pgtable);
> > +}
> > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> > +
> >  /*
> >   * Base infrastructure required to generate basic asces, region, segment,
> >   * and page tables that do not make use of enhanced features like EDAT1.
> 
> Thanks!