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



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