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Date:   Mon, 31 Oct 2022 10:36:08 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Nadav Amit <nadav.amit@gmail.com>, Jann Horn <jannh@google.com>,
        John Hubbard <jhubbard@nvidia.com>, X86 ML <x86@kernel.org>,
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        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        jroedel@suse.de, ubizjak@gmail.com,
        Alistair Popple <apopple@nvidia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment
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On Sun, Oct 30, 2022 at 06:47:23PM -0700, Linus Torvalds wrote:

> Also, it's worth noting that zap_pte_range() does this sanity test:
> 
>                         if (unlikely(page_mapcount(page) < 0))
>                                 print_bad_pte(vma, addr, ptent, page);
> 
> and that is likely worthless now (because it hasn't actually
> decremented the mapcount yet). I didn't remove it, because I wasn't
> sure which option was best:
> 
>  (a) just remove it entirely
> 
>  (b) change the "< 0" to "<= 0"
> 
>  (c) move it to clean_and_free_pages_and_swap_cache() that actually
> does the page_zap_pte_rmap() now.

I'm leaning towards (c); simply because the error case is so terrifying
I feel we should check for it (and I do have vague memories of us
actually hitting something like this in the very distant past).