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.
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