Re: [syzbot] [mm?] BUG: Bad page map (7)

[Yin Fengwei]

Hi Matthew,

On 9/12/23 12:59, Matthew Wilcox wrote:
> On Mon, Sep 11, 2023 at 01:22:51PM -0700, Dave Hansen wrote:
>> On 9/11/23 12:12, Matthew Wilcox wrote:
>>> On Mon, Sep 11, 2023 at 09:55:37AM -0700, Dave Hansen wrote:
>>>> On 9/11/23 09:44, Matthew Wilcox wrote:
>>>>> After fixing your two typos, this assembles to 176 bytes more code than
>>>>> my version.  Not sure that's great.
>>>> Maybe I'm a fool, but 176 bytes of text bloat isn't scaring me off too
>>>> much.  I'd much rather have that than another window into x86 goofiness
>>>> to maintain.
>>>>
>>>> Does that 176 bytes translate into meaningful performance, or is it just
>>>> a bunch of register bit twiddling that the CPU will sail through?
>>> I'm ... not sure how to tell.  It's 1120 bytes vs 944 bytes and crawling
>>> through that much x86 assembly isn't my idea of a great time.  I can
>>> send you objdump -dr for all three options if you like?  Maybe there's
>>> a quick way to compare them that I've never known about.
>>
>> Working patches would be great if you're got 'em handy, plus your
>> .config and generally what compiler you're on.
> 
> gcc (Debian 13.2.0-2) 13.2.0
> 
> I don't think there's anything particularly strange about my .config
> 
> If you compile this patch as-is, you'll get your preferred code.
> Remove the #define DH and you get mine.
> 
> I would say that 176 bytes is 3 cachelines of I$, which isn't free,
> even if all the insns in it can be executed while the CPU is waiting
> for cache misses.  This ought to be a pretty tight loop anyway; we're
> just filling in adjacent PTEs.  There may not be many spare cycles
> for "free" uops to execute.
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index d6ad98ca1288..c9781b8b14af 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -955,6 +955,14 @@ static inline int pte_same(pte_t a, pte_t b)
>  	return a.pte == b.pte;
>  }
>  
> +static inline pte_t pte_next(pte_t pte)
> +{
> +	if (__pte_needs_invert(pte_val(pte)))
> +		return __pte(pte_val(pte) - (1UL << PFN_PTE_SHIFT));
> +	return __pte(pte_val(pte) + (1UL << PFN_PTE_SHIFT));
> +}
> +#define pte_next	pte_next
> +
>  static inline int pte_present(pte_t a)
>  {
>  	return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE);
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 1fba072b3dac..25333cf3c865 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -205,6 +205,10 @@ static inline int pmd_young(pmd_t pmd)
>  #define arch_flush_lazy_mmu_mode()	do {} while (0)
>  #endif
>  
> +#ifndef pte_next
> +#define pte_next(pte)	((pte) + (1UL << PFN_PTE_SHIFT))
> +#endif
> +
>  #ifndef set_ptes
>  /**
>   * set_ptes - Map consecutive pages to a contiguous range of addresses.
> @@ -223,6 +227,11 @@ static inline int pmd_young(pmd_t pmd)
>  static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
>  		pte_t *ptep, pte_t pte, unsigned int nr)
>  {
> +#define DH
> +#ifdef DH
> +	pgprot_t prot = pte_pgprot(pte);
> +	unsigned long pfn = pte_pfn(pte);
> +#endif
>  	page_table_check_ptes_set(mm, ptep, pte, nr);
>  
>  	arch_enter_lazy_mmu_mode();
> @@ -231,7 +240,12 @@ static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
>  		if (--nr == 0)
>  			break;
>  		ptep++;
> -		pte = __pte(pte_val(pte) + (1UL << PFN_PTE_SHIFT));
> +#ifdef DH
> +		pfn++;
> +		pte = pfn_pte(pfn, prot);
> +#else
> +		pte = pte_next(pte);
> +#endif
>  	}
>  	arch_leave_lazy_mmu_mode();
>  }

I checked the commit message of 6b28baca9b1f0d4a42b865da7a05b1c81424bd5c:
    The invert is done by pte/pmd_modify and pfn/pmd/pud_pte for PROTNONE and
    pte/pmd/pud_pfn undo it.
    
    This assume that no code path touches the PFN part of a PTE directly
    without using these primitives.

So maybe we should always use these APIs even we make x86 specific set_ptes()?

I will find a test machine to measure the performance difference of these two
versions by using xfs + will-it-scale. Will keep you guys updated.


Regards
Yin, Fengwei