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Date:   Mon, 29 May 2023 07:36:40 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Matthew Wilcox <willy@infradead.org>
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Subject: Re: [PATCH 05/12] powerpc: add pte_free_defer() for pgtables sharing
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On Mon, 29 May 2023, Matthew Wilcox wrote:
> On Sun, May 28, 2023 at 11:20:21PM -0700, Hugh Dickins wrote:
> > +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> > +{
> > +	struct page *page;
> > +
> > +	page = virt_to_page(pgtable);
> > +	call_rcu(&page->rcu_head, pte_free_now);
> > +}
> 
> This can't be safe (on ppc).  IIRC you might have up to 16x4k page
> tables sharing one 64kB page.  So if you have two page tables from the
> same page being defer-freed simultaneously, you'll reuse the rcu_head
> and I cannot imagine things go well from that point.

Oh yes, of course, thanks for catching that so quickly.
So my s390 and sparc implementations will be equally broken.

> 
> I have no idea how to solve this problem.

I do: I'll have to go back to the more complicated implementation we
actually ran with on powerpc - I was thinking those complications just
related to deposit/withdraw matters, forgetting the one-rcu_head issue.

It uses large (0x10000) increments of the page refcount, avoiding
call_rcu() when already active.

It's not a complication I had wanted to explain or test for now,
but we shall have to.  Should apply equally well to sparc, but s390
more of a problem, since s390 already has its own refcount cleverness.

Thanks, I must dash, out much of the day.

Hugh