Re: [PATCH v12 07/31] mm: make pte_unmap_same compatible with SPF

[Jerome Glisse]

On Tue, Apr 16, 2019 at 03:44:58PM +0200, Laurent Dufour wrote:
> pte_unmap_same() is making the assumption that the page table are still
> around because the mmap_sem is held.
> This is no more the case when running a speculative page fault and
> additional check must be made to ensure that the final page table are still
> there.
> 
> This is now done by calling pte_spinlock() to check for the VMA's
> consistency while locking for the page tables.
> 
> This is requiring passing a vm_fault structure to pte_unmap_same() which is
> containing all the needed parameters.
> 
> As pte_spinlock() may fail in the case of a speculative page fault, if the
> VMA has been touched in our back, pte_unmap_same() should now return 3
> cases :
> 	1. pte are the same (0)
> 	2. pte are different (VM_FAULT_PTNOTSAME)
> 	3. a VMA's changes has been detected (VM_FAULT_RETRY)
> 
> The case 2 is handled by the introduction of a new VM_FAULT flag named
> VM_FAULT_PTNOTSAME which is then trapped in cow_user_page().
> If VM_FAULT_RETRY is returned, it is passed up to the callers to retry the
> page fault while holding the mmap_sem.
> 
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>

Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>


> ---
>  include/linux/mm_types.h |  6 +++++-
>  mm/memory.c              | 37 +++++++++++++++++++++++++++----------
>  2 files changed, 32 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 8ec38b11b361..fd7d38ee2e33 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -652,6 +652,8 @@ typedef __bitwise unsigned int vm_fault_t;
>   * @VM_FAULT_NEEDDSYNC:		->fault did not modify page tables and needs
>   *				fsync() to complete (for synchronous page faults
>   *				in DAX)
> + * @VM_FAULT_PTNOTSAME		Page table entries have changed during a
> + *				speculative page fault handling.
>   * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
>   *
>   */
> @@ -669,6 +671,7 @@ enum vm_fault_reason {
>  	VM_FAULT_FALLBACK       = (__force vm_fault_t)0x000800,
>  	VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
>  	VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
> +	VM_FAULT_PTNOTSAME	= (__force vm_fault_t)0x004000,
>  	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
>  };
>  
> @@ -693,7 +696,8 @@ enum vm_fault_reason {
>  	{ VM_FAULT_RETRY,               "RETRY" },	\
>  	{ VM_FAULT_FALLBACK,            "FALLBACK" },	\
>  	{ VM_FAULT_DONE_COW,            "DONE_COW" },	\
> -	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" }
> +	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },	\
> +	{ VM_FAULT_PTNOTSAME,		"PTNOTSAME" }
>  
>  struct vm_special_mapping {
>  	const char *name;	/* The name, e.g. "[vdso]". */
> diff --git a/mm/memory.c b/mm/memory.c
> index 221ccdf34991..d5bebca47d98 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2094,21 +2094,29 @@ static inline bool pte_map_lock(struct vm_fault *vmf)
>   * parts, do_swap_page must check under lock before unmapping the pte and
>   * proceeding (but do_wp_page is only called after already making such a check;
>   * and do_anonymous_page can safely check later on).
> + *
> + * pte_unmap_same() returns:
> + *	0			if the PTE are the same
> + *	VM_FAULT_PTNOTSAME	if the PTE are different
> + *	VM_FAULT_RETRY		if the VMA has changed in our back during
> + *				a speculative page fault handling.
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> -				pte_t *page_table, pte_t orig_pte)
> +static inline vm_fault_t pte_unmap_same(struct vm_fault *vmf)
>  {
> -	int same = 1;
> +	int ret = 0;
> +
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>  	if (sizeof(pte_t) > sizeof(unsigned long)) {
> -		spinlock_t *ptl = pte_lockptr(mm, pmd);
> -		spin_lock(ptl);
> -		same = pte_same(*page_table, orig_pte);
> -		spin_unlock(ptl);
> +		if (pte_spinlock(vmf)) {
> +			if (!pte_same(*vmf->pte, vmf->orig_pte))
> +				ret = VM_FAULT_PTNOTSAME;
> +			spin_unlock(vmf->ptl);
> +		} else
> +			ret = VM_FAULT_RETRY;
>  	}
>  #endif
> -	pte_unmap(page_table);
> -	return same;
> +	pte_unmap(vmf->pte);
> +	return ret;
>  }
>  
>  static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> @@ -2714,8 +2722,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	int exclusive = 0;
>  	vm_fault_t ret = 0;
>  
> -	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> +	ret = pte_unmap_same(vmf);
> +	if (ret) {
> +		/*
> +		 * If pte != orig_pte, this means another thread did the
> +		 * swap operation in our back.
> +		 * So nothing else to do.
> +		 */
> +		if (ret == VM_FAULT_PTNOTSAME)
> +			ret = 0;
>  		goto out;
> +	}
>  
>  	entry = pte_to_swp_entry(vmf->orig_pte);
>  	if (unlikely(non_swap_entry(entry))) {
> -- 
> 2.21.0
>