Re: [PATCH V5 3/5] arch/powerpc/mm: Nest MMU workaround for mprotect RW upgrade.

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Michael Ellerman <mpe@ellerman.id.au> writes:

> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
>> NestMMU requires us to mark the pte invalid and flush the tlb when we do a
>> RW upgrade of pte. We fixed a variant of this in the fault path in commit
>> Fixes: bd5050e38aec ("powerpc/mm/radix: Change pte relax sequence to handle nest MMU hang")
>
> You don't want the "Fixes:" there.
>
>>
>> Do the same for mprotect upgrades.
>>
>> Hugetlb is handled in the next patch.
>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> ---
>>  arch/powerpc/include/asm/book3s/64/pgtable.h | 18 ++++++++++++++
>>  arch/powerpc/include/asm/book3s/64/radix.h   |  4 ++++
>>  arch/powerpc/mm/pgtable-book3s64.c           | 25 ++++++++++++++++++++
>>  arch/powerpc/mm/pgtable-radix.c              | 18 ++++++++++++++
>>  4 files changed, 65 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 2e6ada28da64..92eaea164700 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -1314,6 +1314,24 @@ static inline int pud_pfn(pud_t pud)
>>  	BUILD_BUG();
>>  	return 0;
>>  }
>
> Can we get a blank line here?
>
>> +#define __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION
>> +pte_t ptep_modify_prot_start(struct vm_area_struct *, unsigned long, pte_t *);
>> +void ptep_modify_prot_commit(struct vm_area_struct *, unsigned long,
>> +			     pte_t *, pte_t, pte_t);
>
> So these are not inline ...
>
>> +/*
>> + * Returns true for a R -> RW upgrade of pte
>> + */
>> +static inline bool is_pte_rw_upgrade(unsigned long old_val, unsigned long new_val)
>> +{
>> +	if (!(old_val & _PAGE_READ))
>> +		return false;
>> +
>> +	if ((!(old_val & _PAGE_WRITE)) && (new_val & _PAGE_WRITE))
>> +		return true;
>> +
>> +	return false;
>> +}
>>  
>>  #endif /* __ASSEMBLY__ */
>>  #endif /* _ASM_POWERPC_BOOK3S_64_PGTABLE_H_ */
>> diff --git a/arch/powerpc/mm/pgtable-book3s64.c b/arch/powerpc/mm/pgtable-book3s64.c
>> index f3c31f5e1026..47c742f002ea 100644
>> --- a/arch/powerpc/mm/pgtable-book3s64.c
>> +++ b/arch/powerpc/mm/pgtable-book3s64.c
>> @@ -400,3 +400,28 @@ void arch_report_meminfo(struct seq_file *m)
>>  		   atomic_long_read(&direct_pages_count[MMU_PAGE_1G]) << 20);
>>  }
>>  #endif /* CONFIG_PROC_FS */
>> +
>> +pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr,
>> +			     pte_t *ptep)
>> +{
>> +	unsigned long pte_val;
>> +
>> +	/*
>> +	 * Clear the _PAGE_PRESENT so that no hardware parallel update is
>> +	 * possible. Also keep the pte_present true so that we don't take
>> +	 * wrong fault.
>> +	 */
>> +	pte_val = pte_update(vma->vm_mm, addr, ptep, _PAGE_PRESENT, _PAGE_INVALID, 0);
>> +
>> +	return __pte(pte_val);
>> +
>> +}
>> +
>> +void ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr,
>> +			     pte_t *ptep, pte_t old_pte, pte_t pte)
>> +{
>
> Which means we're going to be doing a function call to get to here ...
>
>> +	if (radix_enabled())
>> +		return radix__ptep_modify_prot_commit(vma, addr,
>> +						      ptep, old_pte, pte);
>
> And then another function call to get to the radix version ...
>
>> +	set_pte_at(vma->vm_mm, addr, ptep, pte);
>> +}
>> diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
>> index 931156069a81..dced3cd241c2 100644
>> --- a/arch/powerpc/mm/pgtable-radix.c
>> +++ b/arch/powerpc/mm/pgtable-radix.c
>> @@ -1063,3 +1063,21 @@ void radix__ptep_set_access_flags(struct vm_area_struct *vma, pte_t *ptep,
>>  	}
>>  	/* See ptesync comment in radix__set_pte_at */
>>  }
>> +
>> +void radix__ptep_modify_prot_commit(struct vm_area_struct *vma,
>> +				    unsigned long addr, pte_t *ptep,
>> +				    pte_t old_pte, pte_t pte)
>> +{
>> +	struct mm_struct *mm = vma->vm_mm;
>> +
>> +	/*
>> +	 * To avoid NMMU hang while relaxing access we need to flush the tlb before
>> +	 * we set the new value. We need to do this only for radix, because hash
>> +	 * translation does flush when updating the linux pte.
>> +	 */
>> +	if (is_pte_rw_upgrade(pte_val(old_pte), pte_val(pte)) &&
>> +	    (atomic_read(&mm->context.copros) > 0))
>> +		radix__flush_tlb_page(vma, addr);
>
> To finally get here, where we'll realise that 99.99% of processes don't
> use copros and so we have nothing to do except set the PTE.
>
>> +
>> +	set_pte_at(mm, addr, ptep, pte);
>> +}
>
> So can we just make it all inline in the header? Or do we think it's not
> a hot enough path to worry about it?
>

I did try that earlier, But IIRC that didn't work due to header
inclusion issue. I can try that again in an addon patch. That would
require moving things around so that we find different struct
definitions correctly.

-aneesh