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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Thu, 24 Nov 2022 15:08:39 +0100
Message-ID: <CAMuHMdXfCW_2u28cNiuq-6gyda2xA00+04w5v3NFcukc14RyVw@mail.gmail.com>
Subject: Re: [PATCH v2] mm: introduce arch_has_hw_nonleaf_pmd_young()
To:     Juergen Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sander Eikelenboom <linux@eikelenboom.it>,
        Yu Zhao <yuzhao@google.com>
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Hi Jürgen,

On Wed, Nov 23, 2022 at 7:53 AM Juergen Gross <jgross@suse.com> wrote:
> When running as a Xen PV guests commit eed9a328aa1a ("mm: x86: add
> CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG") can cause a protection violation
> in pmdp_test_and_clear_young():
>
>  BUG: unable to handle page fault for address: ffff8880083374d0
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0003) - permissions violation
>  PGD 3026067 P4D 3026067 PUD 3027067 PMD 7fee5067 PTE 8010000008337065
>  Oops: 0003 [#1] PREEMPT SMP NOPTI
>  CPU: 7 PID: 158 Comm: kswapd0 Not tainted 6.1.0-rc5-20221118-doflr+ #1
>  RIP: e030:pmdp_test_and_clear_young+0x25/0x40
>
> This happens because the Xen hypervisor can't emulate direct writes to
> page table entries other than PTEs.
>
> This can easily be fixed by introducing arch_has_hw_nonleaf_pmd_young()
> similar to arch_has_hw_pte_young() and test that instead of
> CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG.
>
> Fixes: eed9a328aa1a ("mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG")
> Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Acked-by: Yu Zhao <yuzhao@google.com>
> Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
> ---
> V2:
> - correct function name in commit message to match patch

Thanks for your patch, which is now commit 3f85e711d5af4fb4 ("mm:
introduce arch_has_hw_nonleaf_pmd_young()") in next-20221124.

noreply@ellerman.id.au reported a build failure for m68k/allmodconfig,
which I have bisected to this commit.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c

> @@ -4073,14 +4073,14 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
>  #endif
>                 walk->mm_stats[MM_NONLEAF_TOTAL]++;
>
> -#ifdef CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG
> -               if (get_cap(LRU_GEN_NONLEAF_YOUNG)) {
> +               if (arch_has_hw_nonleaf_pmd_young() &&
> +                   get_cap(LRU_GEN_NONLEAF_YOUNG)) {
>                         if (!pmd_young(val))

mm/vmscan.c:4102:30: error: implicit declaration of function
'pmd_young'; did you mean 'pte_young'?
[-Werror=implicit-function-declaration]

pmd_young() seems to be defined only on a handful of architectures.

>                                 continue;
>
>                         walk_pmd_range_locked(pud, addr, vma, args, bitmap, &pos);
>                 }
> -#endif
> +
>                 if (!walk->force_scan && !test_bloom_filter(walk->lruvec, walk->max_seq, pmd + i))
>                         continue;
>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds