Re: [PATCH] mm: introduce arch_has_hw_pmd_young()

[Sander Eikelenboom]

On 21/11/2022 10:50, Juergen Gross wrote:
> 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

Hi Juergen,

Thanks for the patch, had it running overnight and did some kernel 
compiles. No crashes with this patch so far, so it seems to work for me.

--
Sander


> > 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);