Re: [PATCH] mm: introduce arch_has_hw_pmd_young()

[Juergen Gross]

On 21.11.22 10:32, Juergen Gross 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_pmd_young()
> similar to arch_has_hw_pte_young() and test that instead of
> CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG.

I just spotted that I missed to update my commit message to match the
patch:

s/arch_has_hw_pmd_young/arch_has_hw_nonleaf_pmd_young/

I'll wait with a resend in order to get some feedback first.


Juergen

> Fixes: eed9a328aa1a ("mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG")
> Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>   arch/x86/include/asm/pgtable.h |  8 ++++++++
>   include/linux/pgtable.h        | 11 +++++++++++
>   mm/vmscan.c                    | 10 +++++-----
>   3 files changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 5059799bebe3..c567a6ed17ce 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1438,6 +1438,14 @@ static inline bool arch_has_hw_pte_young(void)
>   	return true;
>   }
>   
> +#ifdef CONFIG_XEN_PV
> +#define arch_has_hw_nonleaf_pmd_young arch_has_hw_nonleaf_pmd_young
> +static inline bool arch_has_hw_nonleaf_pmd_young(void)
> +{
> +	return !cpu_feature_enabled(X86_FEATURE_XENPV);
> +}
> +#endif
> +
>   #ifdef CONFIG_PAGE_TABLE_CHECK
>   static inline bool pte_user_accessible_page(pte_t pte)
>   {
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index a108b60a6962..58fc7e2d9575 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -260,6 +260,17 @@ static inline int pmdp_clear_flush_young(struct vm_area_struct *vma,
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>   #endif
>   
> +#ifndef arch_has_hw_nonleaf_pmd_young
> +/*
> + * Return whether the accessed bit in non-leaf PMD entries is supported on the
> + * local CPU.
> + */
> +static inline bool arch_has_hw_nonleaf_pmd_young(void)
> +{
> +	return IS_ENABLED(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG);
> +}
> +#endif
> +
>   #ifndef arch_has_hw_pte_young
>   /*
>    * Return whether the accessed bit is supported on the local CPU.
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 04d8b88e5216..a04ac3b18326 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3975,7 +3975,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area
>   			goto next;
>   
>   		if (!pmd_trans_huge(pmd[i])) {
> -			if (IS_ENABLED(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG) &&
> +			if (arch_has_hw_nonleaf_pmd_young() &&
>   			    get_cap(LRU_GEN_NONLEAF_YOUNG))
>   				pmdp_test_and_clear_young(vma, addr, pmd + i);
>   			goto next;
> @@ -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))
>   				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;
>   
> @@ -5354,7 +5354,7 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c
>   	if (arch_has_hw_pte_young() && get_cap(LRU_GEN_MM_WALK))
>   		caps |= BIT(LRU_GEN_MM_WALK);
>   
> -	if (IS_ENABLED(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG) && get_cap(LRU_GEN_NONLEAF_YOUNG))
> +	if (arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG))
>   		caps |= BIT(LRU_GEN_NONLEAF_YOUNG);
>   
>   	return snprintf(buf, PAGE_SIZE, "0x%04x\n", caps);

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