Re: [RFC v5 04/11] mm: VMA sequence count

[Anshuman Khandual]

On 06/16/2017 11:22 PM, Laurent Dufour wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 

First of all, please do mention that its adding a new element into the
vm_area_struct which will act as a sequential lock element and help
in navigating page fault without mmap_sem lock.

> Wrap the VMA modifications (vma_adjust/unmap_page_range) with sequence
> counts such that we can easily test if a VMA is changed

Yeah true.

> 
> The unmap_page_range() one allows us to make assumptions about
> page-tables; when we find the seqcount hasn't changed we can assume
> page-tables are still valid.

Because unmap_page_range() is the only function which can tear it down ?
Or is there any other reason for this assumption ?

> 
> The flip side is that we cannot distinguish between a vma_adjust() and
> the unmap_page_range() -- where with the former we could have
> re-checked the vma bounds against the address.

Distinguished for what purpose ?

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> 
> [port to 4.12 kernel]
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/mm_types.h |  1 +
>  mm/memory.c              |  2 ++
>  mm/mmap.c                | 13 +++++++++++++
>  3 files changed, 16 insertions(+)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 45cdb27791a3..8945743e4609 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -342,6 +342,7 @@ struct vm_area_struct {
>  	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
>  #endif
>  	struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> +	seqcount_t vm_sequence;
>  };
>  
>  struct core_thread {
> diff --git a/mm/memory.c b/mm/memory.c
> index f1132f7931ef..5d259cd67a83 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1379,6 +1379,7 @@ void unmap_page_range(struct mmu_gather *tlb,
>  	unsigned long next;
>  
>  	BUG_ON(addr >= end);
> +	write_seqcount_begin(&vma->vm_sequence);
>  	tlb_start_vma(tlb, vma);
>  	pgd = pgd_offset(vma->vm_mm, addr);
>  	do {
> @@ -1388,6 +1389,7 @@ void unmap_page_range(struct mmu_gather *tlb,
>  		next = zap_p4d_range(tlb, vma, pgd, addr, next, details);
>  	} while (pgd++, addr = next, addr != end);
>  	tlb_end_vma(tlb, vma);
> +	write_seqcount_end(&vma->vm_sequence);
>  }
>  
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f82741e199c0..9f86356d0012 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -543,6 +543,8 @@ void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
>  	else
>  		mm->highest_vm_end = vma->vm_end;
>  
> +	seqcount_init(&vma->vm_sequence);
> +
>  	/*
>  	 * vma->vm_prev wasn't known when we followed the rbtree to find the
>  	 * correct insertion point for that vma. As a result, we could not
> @@ -677,6 +679,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  	long adjust_next = 0;
>  	int remove_next = 0;
>  
> +	write_seqcount_begin(&vma->vm_sequence);
> +	if (next)
> +		write_seqcount_begin_nested(&next->vm_sequence, SINGLE_DEPTH_NESTING);
> +
>  	if (next && !insert) {
>  		struct vm_area_struct *exporter = NULL, *importer = NULL;
>  
> @@ -888,6 +894,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  		mm->map_count--;
>  		mpol_put(vma_policy(next));
>  		kmem_cache_free(vm_area_cachep, next);
> +		write_seqcount_end(&next->vm_sequence);
>  		/*
>  		 * In mprotect's case 6 (see comments on vma_merge),
>  		 * we must remove another next too. It would clutter
> @@ -901,6 +908,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  			 * "vma->vm_next" gap must be updated.
>  			 */
>  			next = vma->vm_next;
> +			if (next)
> +				write_seqcount_begin_nested(&next->vm_sequence, SINGLE_DEPTH_NESTING);
>  		} else {
>  			/*
>  			 * For the scope of the comment "next" and
> @@ -947,6 +956,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  	if (insert && file)
>  		uprobe_mmap(insert);
>  
> +	if (next)
> +		write_seqcount_end(&next->vm_sequence);
> +	write_seqcount_end(&vma->vm_sequence)
> +
>  	validate_mm(mm);
>  
>  	return 0;

Why we are changing the sequence for 'next' element here as well ?
Is this because next VMA may be modified during the __vma_adjust()
process ? Just out of curiosity.