Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Balbir Singh]

On Mon, 2016-02-15 at 16:31 +0530, Aneesh Kumar K.V wrote:
> Balbir Singh  writes:
> 
> > > Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
> > > can happen outside that. Now i had a variant for kick_all_cpus_sync that
> > > ignored idle cpus. But then that needs more verification.
> > > 
> > > http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105
> > Can be racy as a CPU moves from non-idle to idle
> > 
> > In
> > 
> > > + pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
> > > + /*
> > > +  * This ensures that generic code that rely on IRQ disabling
> > > +  * to prevent a parallel THP split work as expected.
> > > +  */
> > > + kick_all_cpus_sync();
> > 
> > pmdp_invalidate()->pmd_hugepage_update() can still run in parallel with 
> > find_linux_pte_or_hugepte() and race.. Am I missing something?
> > 
> 
> Yes. But then we make sure that the pte_t returned by
> find_linux_pte_or_hugepte doesn't change to a regular pmd entry by using
> that kick. Now callers of find_lnux_pte_or_hugepte will check for
> _PAGE_PRESENT. So if it called before
> pmd_hugepage_update(_PAGE_PRESENT), we wait for the caller to finish the
> usage (via kick()). Or they bail out after finding _PAGE_PRESENT cleared.

Makes sense, but I would still check the assumption about checking for
_PAGE_PRESENT

Balbir Singh
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Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Aneesh Kumar K.V]

Balbir Singh  writes:

>> Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
>> can happen outside that. Now i had a variant for kick_all_cpus_sync that
>> ignored idle cpus. But then that needs more verification.
>> 
>> http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105
> Can be racy as a CPU moves from non-idle to idle
>
> In
>
>> + pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
>> + /*
>> +  * This ensures that generic code that rely on IRQ disabling
>> +  * to prevent a parallel THP split work as expected.
>> +  */
>> + kick_all_cpus_sync();
>
> pmdp_invalidate()->pmd_hugepage_update() can still run in parallel with 
> find_linux_pte_or_hugepte() and race.. Am I missing something?
>

Yes. But then we make sure that the pte_t returned by
find_linux_pte_or_hugepte doesn't change to a regular pmd entry by using
that kick. Now callers of find_lnux_pte_or_hugepte will check for
_PAGE_PRESENT. So if it called before
pmd_hugepage_update(_PAGE_PRESENT), we wait for the caller to finish the
usage (via kick()). Or they bail out after finding _PAGE_PRESENT cleared.

-aneesh

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Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Balbir Singh]

> Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
> can happen outside that. Now i had a variant for kick_all_cpus_sync that
> ignored idle cpus. But then that needs more verification.
> 
> http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105
Can be racy as a CPU moves from non-idle to idle

In

> + pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
> + /*
> +  * This ensures that generic code that rely on IRQ disabling
> +  * to prevent a parallel THP split work as expected.
> +  */
> + kick_all_cpus_sync();

pmdp_invalidate()->pmd_hugepage_update() can still run in parallel with 
find_linux_pte_or_hugepte() and race.. Am I missing something?

Balbir Singh


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Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Aneesh Kumar K.V]

Balbir Singh  writes:

> On Tue, 2016-02-09 at 06:50 +0530, Aneesh Kumar K.V wrote:
>> 
>> Also make sure we wait for irq disable section in other cpus to finish
>> before flipping a huge pte entry with a regular pmd entry. Code paths
>> like find_linux_pte_or_hugepte depend on irq disable to get
>> a stable pte_t pointer. A parallel thp split need to make sure we
>> don't convert a pmd pte to a regular pmd entry without waiting for the
>> irq disable section to finish.
>> 
>> Acked-by: Kirill A. Shutemov 
>> Signed-off-by: Aneesh Kumar K.V 
>> ---
>>  arch/powerpc/include/asm/book3s/64/pgtable.h |  4 
>>  arch/powerpc/mm/pgtable_64.c | 35
>> +++-
>>  include/asm-generic/pgtable.h|  8 +++
>>  mm/huge_memory.c |  1 +
>>  4 files changed, 47 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 8d1c41d28318..ac07a30a7934 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -281,6 +281,10 @@ extern pgtable_t pgtable_trans_huge_withdraw(struct
>> mm_struct *mm, pmd_t *pmdp);
>>  extern void pmdp_invalidate(struct vm_area_struct *vma, unsigned long
>> address,
>>  pmd_t *pmdp);
>>  
>> +#define __HAVE_ARCH_PMDP_HUGE_SPLIT_PREPARE
>> +extern void pmdp_huge_split_prepare(struct vm_area_struct *vma,
>> +unsigned long address, pmd_t *pmdp);
>> +
>>  #define pmd_move_must_withdraw pmd_move_must_withdraw
>>  struct spinlock;
>>  static inline int pmd_move_must_withdraw(struct spinlock *new_pmd_ptl,
>> diff --git a/arch/powerpc/mm/pgtable_64.c b/arch/powerpc/mm/pgtable_64.c
>> index 3124a20d0fab..c8a00da39969 100644
>> --- a/arch/powerpc/mm/pgtable_64.c
>> +++ b/arch/powerpc/mm/pgtable_64.c
>> @@ -646,6 +646,30 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct
>> *mm, pmd_t *pmdp)
>>  return pgtable;
>>  }
>>  
>> +void pmdp_huge_split_prepare(struct vm_area_struct *vma,
>> + unsigned long address, pmd_t *pmdp)
>> +{
>> +VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> +
>> +#ifdef CONFIG_DEBUG_VM
>> +BUG_ON(REGION_ID(address) != USER_REGION_ID);
>> +#endif
>> +/*
>> + * We can't mark the pmd none here, because that will cause a race
>> + * against exit_mmap. We need to continue mark pmd TRANS HUGE, while
>> + * we spilt, but at the same time we wan't rest of the ppc64 code
>> + * not to insert hash pte on this, because we will be modifying
>> + * the deposited pgtable in the caller of this function. Hence
>> + * clear the _PAGE_USER so that we move the fault handling to
>> + * higher level function and that will serialize against ptl.
>> + * We need to flush existing hash pte entries here even though,
>> + * the translation is still valid, because we will withdraw
>> + * pgtable_t after this.
>> + */
>> +pmd_hugepage_update(vma->vm_mm, address, pmdp, _PAGE_USER, 0);
>
> Can this break any checks for _PAGE_USER? From other paths?


Should not, that is the same condition we use for autonuma.

>
>> +}
>> +
>> +
>>  /*
>>   * set a new huge pmd. We should not be called for updating
>>   * an existing pmd entry. That should go via pmd_hugepage_update.
>> @@ -663,10 +687,19 @@ void set_pmd_at(struct mm_struct *mm, unsigned long
>> addr,
>>  return set_pte_at(mm, addr, pmdp_ptep(pmdp), pmd_pte(pmd));
>>  }
>>  
>> +/*
>> + * We use this to invalidate a pmdp entry before switching from a
>> + * hugepte to regular pmd entry.
>> + */
>>  void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>>   pmd_t *pmdp)
>>  {
>> -pmd_hugepage_update(vma->vm_mm, address, pmdp, _PAGE_PRESENT, 0);
>> +pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
>> +/*
>> + * This ensures that generic code that rely on IRQ disabling
>> + * to prevent a parallel THP split work as expected.
>> + */
>> +kick_all_cpus_sync();
>
> Seems expensive, anyway I think the right should do something like or a 
> wrapper
> for it
>
> on_each_cpu_mask(mm_cpumask(vma->vm_mm), do_nothing, NULL, 1);
>
> do_nothing is not exported, but that can be fixed :)
>

Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
can happen outside that. Now i had a variant for kick_all_cpus_sync that
ignored idle cpus. But then that needs more verification.

http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105

-aneesh

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Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Balbir Singh]

On Tue, 2016-02-09 at 06:50 +0530, Aneesh Kumar K.V wrote:
> 
> Also make sure we wait for irq disable section in other cpus to finish
> before flipping a huge pte entry with a regular pmd entry. Code paths
> like find_linux_pte_or_hugepte depend on irq disable to get
> a stable pte_t pointer. A parallel thp split need to make sure we
> don't convert a pmd pte to a regular pmd entry without waiting for the
> irq disable section to finish.
> 
> Acked-by: Kirill A. Shutemov 
> Signed-off-by: Aneesh Kumar K.V 
> ---
>  arch/powerpc/include/asm/book3s/64/pgtable.h |  4 
>  arch/powerpc/mm/pgtable_64.c | 35
> +++-
>  include/asm-generic/pgtable.h|  8 +++
>  mm/huge_memory.c |  1 +
>  4 files changed, 47 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h
> b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index 8d1c41d28318..ac07a30a7934 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -281,6 +281,10 @@ extern pgtable_t pgtable_trans_huge_withdraw(struct
> mm_struct *mm, pmd_t *pmdp);
>  extern void pmdp_invalidate(struct vm_area_struct *vma, unsigned long
> address,
>   pmd_t *pmdp);
>  
> +#define __HAVE_ARCH_PMDP_HUGE_SPLIT_PREPARE
> +extern void pmdp_huge_split_prepare(struct vm_area_struct *vma,
> + unsigned long address, pmd_t *pmdp);
> +
>  #define pmd_move_must_withdraw pmd_move_must_withdraw
>  struct spinlock;
>  static inline int pmd_move_must_withdraw(struct spinlock *new_pmd_ptl,
> diff --git a/arch/powerpc/mm/pgtable_64.c b/arch/powerpc/mm/pgtable_64.c
> index 3124a20d0fab..c8a00da39969 100644
> --- a/arch/powerpc/mm/pgtable_64.c
> +++ b/arch/powerpc/mm/pgtable_64.c
> @@ -646,6 +646,30 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct
> *mm, pmd_t *pmdp)
>   return pgtable;
>  }
>  
> +void pmdp_huge_split_prepare(struct vm_area_struct *vma,
> +  unsigned long address, pmd_t *pmdp)
> +{
> + VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> +
> +#ifdef CONFIG_DEBUG_VM
> + BUG_ON(REGION_ID(address) != USER_REGION_ID);
> +#endif
> + /*
> +  * We can't mark the pmd none here, because that will cause a race
> +  * against exit_mmap. We need to continue mark pmd TRANS HUGE, while
> +  * we spilt, but at the same time we wan't rest of the ppc64 code
> +  * not to insert hash pte on this, because we will be modifying
> +  * the deposited pgtable in the caller of this function. Hence
> +  * clear the _PAGE_USER so that we move the fault handling to
> +  * higher level function and that will serialize against ptl.
> +  * We need to flush existing hash pte entries here even though,
> +  * the translation is still valid, because we will withdraw
> +  * pgtable_t after this.
> +  */
> + pmd_hugepage_update(vma->vm_mm, address, pmdp, _PAGE_USER, 0);

Can this break any checks for _PAGE_USER? From other paths?

> +}
> +
> +
>  /*
>   * set a new huge pmd. We should not be called for updating
>   * an existing pmd entry. That should go via pmd_hugepage_update.
> @@ -663,10 +687,19 @@ void set_pmd_at(struct mm_struct *mm, unsigned long
> addr,
>   return set_pte_at(mm, addr, pmdp_ptep(pmdp), pmd_pte(pmd));
>  }
>  
> +/*
> + * We use this to invalidate a pmdp entry before switching from a
> + * hugepte to regular pmd entry.
> + */
>  void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>    pmd_t *pmdp)
>  {
> - pmd_hugepage_update(vma->vm_mm, address, pmdp, _PAGE_PRESENT, 0);
> + pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
> + /*
> +  * This ensures that generic code that rely on IRQ disabling
> +  * to prevent a parallel THP split work as expected.
> +  */
> + kick_all_cpus_sync();

Seems expensive, anyway I think the right should do something like or a wrapper
for it

on_each_cpu_mask(mm_cpumask(vma->vm_mm), do_nothing, NULL, 1);

do_nothing is not exported, but that can be fixed :)

Balbir Singh
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