linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Message-ID: <Y1+XCALog8bW7Hgl@hirez.programming.kicks-ass.net>
Date:   Mon, 31 Oct 2022 10:36:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nadav Amit <nadav.amit@...il.com>, Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>, X86 ML <x86@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jroedel@...e.de, ubizjak@...il.com,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

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).