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Date:   Mon, 29 May 2023 15:02:02 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
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Subject: Re: [PATCH 05/12] powerpc: add pte_free_defer() for pgtables sharing
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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.

I have no idea how to solve this problem.