linux-kernel - Re: [RFC v5 03/11] mm: Introduce pte_spinlock for FAULT_FLAG

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Date:   Tue, 8 Aug 2017 16:05:07 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC v5 03/11] mm: Introduce pte_spinlock for
 FAULT_FLAG_SPECULATIVE

On 06/16/2017 11:22 PM, Laurent Dufour wrote:
> When handling page fault without holding the mmap_sem the fetch of the
> pte lock pointer and the locking will have to be done while ensuring
> that the VMA is not touched in our back.

It does not change things from whats happening right now, where do we
check that VMA has not changed by now ?

> 
> So move the fetch and locking operations in a dedicated function.
> 
> Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
> ---
>  mm/memory.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 40834444ea0d..f1132f7931ef 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2240,6 +2240,13 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
>  }
>  
> +static bool pte_spinlock(struct vm_fault *vmf)
> +{
> +	vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> +	spin_lock(vmf->ptl);
> +	return true;
> +}
> +

Moving them together makes sense but again if blocks are redundant when
it returns true all the time.

>  static bool pte_map_lock(struct vm_fault *vmf)
>  {
>  	vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, vmf->address, &vmf->ptl);
> @@ -3552,8 +3559,8 @@ static int do_numa_page(struct vm_fault *vmf)
>  	 * validation through pte_unmap_same(). It's of NUMA type but
>  	 * the pfn may be screwed if the read is non atomic.
>  	 */
> -	vmf->ptl = pte_lockptr(vma->vm_mm, vmf->pmd);
> -	spin_lock(vmf->ptl);
> +	if (!pte_spinlock(vmf))
> +		return VM_FAULT_RETRY;
>  	if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
>  		goto out;
> @@ -3745,8 +3752,8 @@ static int handle_pte_fault(struct vm_fault *vmf)
>  	if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
>  		return do_numa_page(vmf);
>  
> -	vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> -	spin_lock(vmf->ptl);
> +	if (!pte_spinlock(vmf))
> +		return VM_FAULT_RETRY;
>  	entry = vmf->orig_pte;
>  	if (unlikely(!pte_same(*vmf->pte, entry)))
>  		goto unlock;
>