Re: [PATCH V2] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Kirill A. Shutemov]

On Mon, Feb 08, 2016 at 11:44:22AM +0530, Aneesh Kumar K.V wrote:
> With ppc64 we use the deposited pgtable_t to store the hash pte slot
> information. We should not withdraw the deposited pgtable_t without
> marking the pmd none. This ensure that low level hash fault handling
> will skip this huge pte and we will handle them at upper levels.
> 
> Recent change to pmd splitting changed the above in order to handle the
> race between pmd split and exit_mmap. The race is explained below.
> 
> Consider following race:
> 
> 		CPU0				CPU1
> shrink_page_list()
>   add_to_swap()
>     split_huge_page_to_list()
>       __split_huge_pmd_locked()
>         pmdp_huge_clear_flush_notify()
> 	// pmd_none() == true
> 					exit_mmap()
> 					  unmap_vmas()
> 					    zap_pmd_range()
> 					      // no action on pmd since pmd_none() == true
> 	pmd_populate()
> 
> As result the THP will not be freed. The leak is detected by check_mm():
> 
> 	BUG: Bad rss-counter state mm:ffff880058d2e580 idx:1 val:512
> 
> The above required us to not mark pmd none during a pmd split.
> 
> The fix for ppc is to clear the huge pte of _PAGE_USER, so that low
> level fault handling code skip this pte. At higher level we do take ptl
> lock. That should serialze us against the pmd split. Once the lock is
> acquired we do check the pmd again using pmd_same. That should always
> return false for us and hence we should retry the access.

I guess it worth mention that this serialization against ptl happens in
huge_pmd_set_accessed(), if I didn't miss anything.

> 
> 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.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> ---
>  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..0415856941e0 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_SPLITTING_FLUSH
> +extern void pmdp_huge_splitting_flush(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..e8214b7f2210 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_splitting_flush(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);
> +}
> +
> +
>  /*
>   * 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();
>  }
>  
>  /*
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index 0b3c0d39ef75..93a0937652ec 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -239,6 +239,14 @@ extern void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>  			    pmd_t *pmdp);
>  #endif
>  
> +#ifndef __HAVE_ARCH_PMDP_HUGE_SPLITTING_FLUSH
> +static inline void pmdp_huge_splitting_flush(struct vm_area_struct *vma,
> +					     unsigned long address, pmd_t *pmdp)
> +{
> +
> +}
> +#endif
> +
>  #ifndef __HAVE_ARCH_PTE_SAME
>  static inline int pte_same(pte_t pte_a, pte_t pte_b)
>  {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 36c070167b71..b52d16a86e91 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2860,6 +2860,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	young = pmd_young(*pmd);
>  	dirty = pmd_dirty(*pmd);
>  
> +	pmdp_huge_splitting_flush(vma, haddr, pmd);

Let's call it pmdp_huge_split_prepare().

"_flush" part is ppc-specific implementation detail and generic code
should not expect tlb to be flushed there.

Otherwise,

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

>  	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
>  	pmd_populate(mm, &_pmd, pgtable);
>  
> -- 
> 2.5.0
> 
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-- 
 Kirill A. Shutemov