Subject: Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

From: Jerome Glisse <[email-masked]>
Date: Tue, 3 Apr 2018 15:10:05 -0400

On Tue, Mar 13, 2018 at 06:59:36PM +0100, 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.
> 
> Signed-off-by: Laurent Dufour <[email-masked]>
> ---
>  include/linux/mm.h |  1 +
>  mm/memory.c        | 29 +++++++++++++++++++----------
>  2 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2f3e98edc94a..b6432a261e63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1199,6 +1199,7 @@ static inline void clear_page_pfmemalloc(struct page *page)
>  #define VM_FAULT_NEEDDSYNC  0x2000	/* ->fault did not modify page tables
>  					 * and needs fsync() to complete (for
>  					 * synchronous page faults in DAX) */
> +#define VM_FAULT_PTNOTSAME 0x4000	/* Page table entries have changed */
>  
>  #define VM_FAULT_ERROR	(VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
>  			 VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
> diff --git a/mm/memory.c b/mm/memory.c
> index 21b1212a0892..4bc7b0bdcb40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2309,21 +2309,29 @@ static 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 int 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)
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>  	int exclusive = 0;
>  	int ret = 0;
>  
> -	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> +	ret = pte_unmap_same(vmf);
> +	if (ret)
>  		goto out;
>  

This change what do_swap_page() returns ie before it was returning 0
when locked pte lookup was different from orig_pte. After this patch
it returns VM_FAULT_PTNOTSAME but this is a new return value for
handle_mm_fault() (the do_swap_page() return value is what ultimately
get return by handle_mm_fault())

Do we really want that ? This might confuse some existing user of
handle_mm_fault() and i am not sure of the value of that information
to caller.

Note i do understand that you want to return retry if anything did
change from underneath and thus need to differentiate from when the
pte value are not the same.

Cheers,
Jérôme