Re: [PATCH V3] mm/thp: Split out pmd collpase flush into a separate functions

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

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> On Mon, May 11, 2015 at 12:09:30PM +0530, Aneesh Kumar K.V wrote:
>> Architectures like ppc64 [1] need to do special things while clearing
>> pmd before a collapse. For them this operation is largely different
>> from a normal hugepage pte clear. Hence add a separate function
>> to clear pmd before collapse. After this patch pmdp_* functions
>> operate only on hugepage pte, and not on regular pmd_t values
>> pointing to page table.
>> 
>> [1] ppc64 needs to invalidate all the normal page pte mappings we
>> already have inserted in the hardware hash page table. But before
>> doing that we need to make sure there are no parallel hash page
>> table insert going on. So we need to do a kick_all_cpus_sync()
>> before flushing the older hash table entries. By moving this to
>> a separate function we capture these details and mention how it
>> is different from a hugepage pte clear.
>> 
>> This patch is a cleanup and only does code movement for clarity.
>> There should not be any change in functionality.
>> 
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>> ---
>> Changes from V2:
>> * Update commit message
>> * Address review feedback
>> 
>>  arch/powerpc/include/asm/pgtable-ppc64.h |  4 ++
>>  arch/powerpc/mm/pgtable_64.c             | 76 +++++++++++++++++---------------
>>  include/asm-generic/pgtable.h            | 19 ++++++++
>>  mm/huge_memory.c                         |  2 +-
>>  4 files changed, 65 insertions(+), 36 deletions(-)
>> 
>> diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
>> index 43e6ad424c7f..f5b98b2a45f0 100644
>> --- a/arch/powerpc/include/asm/pgtable-ppc64.h
>> +++ b/arch/powerpc/include/asm/pgtable-ppc64.h
>> @@ -576,6 +576,10 @@ static inline void pmdp_set_wrprotect(struct mm_struct *mm, unsigned long addr,
>>  extern void pmdp_splitting_flush(struct vm_area_struct *vma,
>>  				 unsigned long address, pmd_t *pmdp);
>>  
>> +#define pmd_collapse_flush pmd_collapse_flush
>> +extern pmd_t pmd_collapse_flush(struct vm_area_struct *vma,
>> +				unsigned long address, pmd_t *pmdp);
>> +
>>  #define __HAVE_ARCH_PGTABLE_DEPOSIT
>>  extern void pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp,
>>  				       pgtable_t pgtable);
>> diff --git a/arch/powerpc/mm/pgtable_64.c b/arch/powerpc/mm/pgtable_64.c
>> index 59daa5eeec25..b651179ac4da 100644
>> --- a/arch/powerpc/mm/pgtable_64.c
>> +++ b/arch/powerpc/mm/pgtable_64.c
>> @@ -560,41 +560,47 @@ pmd_t pmdp_clear_flush(struct vm_area_struct *vma, unsigned long address,
>>  	pmd_t pmd;
>>  
>>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> -	if (pmd_trans_huge(*pmdp)) {
>> -		pmd = pmdp_get_and_clear(vma->vm_mm, address, pmdp);
>> -	} else {
>> -		/*
>> -		 * khugepaged calls this for normal pmd
>> -		 */
>> -		pmd = *pmdp;
>> -		pmd_clear(pmdp);
>> -		/*
>> -		 * Wait for all pending hash_page to finish. This is needed
>> -		 * in case of subpage collapse. When we collapse normal pages
>> -		 * to hugepage, we first clear the pmd, then invalidate all
>> -		 * the PTE entries. The assumption here is that any low level
>> -		 * page fault will see a none pmd and take the slow path that
>> -		 * will wait on mmap_sem. But we could very well be in a
>> -		 * hash_page with local ptep pointer value. Such a hash page
>> -		 * can result in adding new HPTE entries for normal subpages.
>> -		 * That means we could be modifying the page content as we
>> -		 * copy them to a huge page. So wait for parallel hash_page
>> -		 * to finish before invalidating HPTE entries. We can do this
>> -		 * by sending an IPI to all the cpus and executing a dummy
>> -		 * function there.
>> -		 */
>> -		kick_all_cpus_sync();
>> -		/*
>> -		 * Now invalidate the hpte entries in the range
>> -		 * covered by pmd. This make sure we take a
>> -		 * fault and will find the pmd as none, which will
>> -		 * result in a major fault which takes mmap_sem and
>> -		 * hence wait for collapse to complete. Without this
>> -		 * the __collapse_huge_page_copy can result in copying
>> -		 * the old content.
>> -		 */
>> -		flush_tlb_pmd_range(vma->vm_mm, &pmd, address);
>> -	}
>> +	VM_BUG_ON(!pmd_trans_huge(*pmdp));
>> +	pmd = pmdp_get_and_clear(vma->vm_mm, address, pmdp);
>> +	return pmd;
>
> Looks like with this cange you don't need Power-specific
> pmdp_clear_flush() -- generic one would work for you.

That is correct. Will update the series and do that as a follow up patch ?

>
> It seems you want change semantics of pmdp_clear_flush(): it should be
> called only for huge pmds. I'm fine with that. But we need at least
> document that. And probably rename the helper to reflect semantics:
> pmdp_huge_clear_flush()?
>

So you are suggesting to callout _huge_ in the APIs. Will fix. Also will
do a cleanup to update other functions ?

> And we need  VM_BUG_ON(!pmd_trans_huge(*pmdp)) in generic helper too.
>
> What about pmdp_clear_flush_young()? Should we change it the same way?
>

ok

>> +}
>> +
>> +pmd_t pmd_collapse_flush(struct vm_area_struct *vma, unsigned long address,
>> +			 pmd_t *pmdp)
>> +{
>> +	pmd_t pmd;
>> +
>> +	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> +	VM_BUG_ON(pmd_trans_huge(*pmdp));
>> +
>> +	pmd = *pmdp;
>> +	pmd_clear(pmdp);
>> +	/*
>> +	 * Wait for all pending hash_page to finish. This is needed
>> +	 * in case of subpage collapse. When we collapse normal pages
>> +	 * to hugepage, we first clear the pmd, then invalidate all
>> +	 * the PTE entries. The assumption here is that any low level
>> +	 * page fault will see a none pmd and take the slow path that
>> +	 * will wait on mmap_sem. But we could very well be in a
>> +	 * hash_page with local ptep pointer value. Such a hash page
>> +	 * can result in adding new HPTE entries for normal subpages.
>> +	 * That means we could be modifying the page content as we
>> +	 * copy them to a huge page. So wait for parallel hash_page
>> +	 * to finish before invalidating HPTE entries. We can do this
>> +	 * by sending an IPI to all the cpus and executing a dummy
>> +	 * function there.
>> +	 */
>> +	kick_all_cpus_sync();
>> +	/*
>> +	 * Now invalidate the hpte entries in the range
>> +	 * covered by pmd. This make sure we take a
>> +	 * fault and will find the pmd as none, which will
>> +	 * result in a major fault which takes mmap_sem and
>> +	 * hence wait for collapse to complete. Without this
>> +	 * the __collapse_huge_page_copy can result in copying
>> +	 * the old content.
>> +	 */
>> +	flush_tlb_pmd_range(vma->vm_mm, &pmd, address);
>>  	return pmd;
>>  }
>>  
>> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
>> index 39f1d6a2b04d..edc90a2261f7 100644
>> --- a/include/asm-generic/pgtable.h
>> +++ b/include/asm-generic/pgtable.h
>> @@ -189,6 +189,25 @@ extern void pmdp_splitting_flush(struct vm_area_struct *vma,
>>  				 unsigned long address, pmd_t *pmdp);
>>  #endif
>>  
>> +#ifndef pmd_collapse_flush
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +static inline pmd_t pmd_collapse_flush(struct vm_area_struct *vma,
>> +				       unsigned long address,
>> +				       pmd_t *pmdp)
>> +{
>> +	return pmdp_clear_flush(vma, address, pmdp);
>> +}
>> +#else
>> +static inline pmd_t pmd_collapse_flush(struct vm_area_struct *vma,
>> +				       unsigned long address,
>> +				       pmd_t *pmdp)
>> +{
>> +	BUILD_BUG();
>> +	return __pmd(0);
>> +}
>> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>> +#endif
>> +
>>  #ifndef __HAVE_ARCH_PGTABLE_DEPOSIT
>>  extern void pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp,
>>  				       pgtable_t pgtable);
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 078832cf3636..009a5de619fd 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2499,7 +2499,7 @@ static void collapse_huge_page(struct mm_struct *mm,
>>  	 * huge and small TLB entries for the same virtual address
>>  	 * to avoid the risk of CPU bugs in that area.
>>  	 */
>> -	_pmd = pmdp_clear_flush(vma, address, pmd);
>> +	_pmd = pmd_collapse_flush(vma, address, pmd);
>
> Let's name it pmdp_collapse_flush().
> We are not hugely consistent on pmd vs. pmdp, but we have
> pmdp_splittitng_flush() counterpart.
>

ok 


>>  	spin_unlock(pmd_ptl);
>>  	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
>>  

-aneesh