Re: [PATCH v12 17/31] mm: introduce __page_add_new_anon_rmap()

[Jerome Glisse <jglisse@redhat.com>]

On Tue, Apr 16, 2019 at 03:45:08PM +0200, Laurent Dufour wrote:
> When dealing with speculative page fault handler, we may race with VMA
> being split or merged. In this case the vma->vm_start and vm->vm_end
> fields may not match the address the page fault is occurring.
> 
> This can only happens when the VMA is split but in that case, the
> anon_vma pointer of the new VMA will be the same as the original one,
> because in __split_vma the new->anon_vma is set to src->anon_vma when
> *new = *vma.
> 
> So even if the VMA boundaries are not correct, the anon_vma pointer is
> still valid.
> 
> If the VMA has been merged, then the VMA in which it has been merged
> must have the same anon_vma pointer otherwise the merge can't be done.
> 
> So in all the case we know that the anon_vma is valid, since we have
> checked before starting the speculative page fault that the anon_vma
> pointer is valid for this VMA and since there is an anon_vma this
> means that at one time a page has been backed and that before the VMA
> is cleaned, the page table lock would have to be grab to clean the
> PTE, and the anon_vma field is checked once the PTE is locked.
> 
> This patch introduce a new __page_add_new_anon_rmap() service which
> doesn't check for the VMA boundaries, and create a new inline one
> which do the check.
> 
> When called from a page fault handler, if this is not a speculative one,
> there is a guarantee that vm_start and vm_end match the faulting address,
> so this check is useless. In the context of the speculative page fault
> handler, this check may be wrong but anon_vma is still valid as explained
> above.
> 
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

> ---
>  include/linux/rmap.h | 12 ++++++++++--
>  mm/memory.c          |  8 ++++----
>  mm/rmap.c            |  5 ++---
>  3 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 988d176472df..a5d282573093 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -174,8 +174,16 @@ void page_add_anon_rmap(struct page *, struct vm_area_struct *,
>  		unsigned long, bool);
>  void do_page_add_anon_rmap(struct page *, struct vm_area_struct *,
>  			   unsigned long, int);
> -void page_add_new_anon_rmap(struct page *, struct vm_area_struct *,
> -		unsigned long, bool);
> +void __page_add_new_anon_rmap(struct page *, struct vm_area_struct *,
> +			      unsigned long, bool);
> +static inline void page_add_new_anon_rmap(struct page *page,
> +					  struct vm_area_struct *vma,
> +					  unsigned long address, bool compound)
> +{
> +	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
> +	__page_add_new_anon_rmap(page, vma, address, compound);
> +}
> +
>  void page_add_file_rmap(struct page *, bool);
>  void page_remove_rmap(struct page *, bool);
>  
> diff --git a/mm/memory.c b/mm/memory.c
> index be93f2c8ebe0..46f877b6abea 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2347,7 +2347,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  		 * thread doing COW.
>  		 */
>  		ptep_clear_flush_notify(vma, vmf->address, vmf->pte);
> -		page_add_new_anon_rmap(new_page, vma, vmf->address, false);
> +		__page_add_new_anon_rmap(new_page, vma, vmf->address, false);
>  		mem_cgroup_commit_charge(new_page, memcg, false, false);
>  		__lru_cache_add_active_or_unevictable(new_page, vmf->vma_flags);
>  		/*
> @@ -2897,7 +2897,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  
>  	/* ksm created a completely new copy */
>  	if (unlikely(page != swapcache && swapcache)) {
> -		page_add_new_anon_rmap(page, vma, vmf->address, false);
> +		__page_add_new_anon_rmap(page, vma, vmf->address, false);
>  		mem_cgroup_commit_charge(page, memcg, false, false);
>  		__lru_cache_add_active_or_unevictable(page, vmf->vma_flags);
>  	} else {
> @@ -3049,7 +3049,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	}
>  
>  	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
> -	page_add_new_anon_rmap(page, vma, vmf->address, false);
> +	__page_add_new_anon_rmap(page, vma, vmf->address, false);
>  	mem_cgroup_commit_charge(page, memcg, false, false);
>  	__lru_cache_add_active_or_unevictable(page, vmf->vma_flags);
>  setpte:
> @@ -3328,7 +3328,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>  	/* copy-on-write page */
>  	if (write && !(vmf->vma_flags & VM_SHARED)) {
>  		inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
> -		page_add_new_anon_rmap(page, vma, vmf->address, false);
> +		__page_add_new_anon_rmap(page, vma, vmf->address, false);
>  		mem_cgroup_commit_charge(page, memcg, false, false);
>  		__lru_cache_add_active_or_unevictable(page, vmf->vma_flags);
>  	} else {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index e5dfe2ae6b0d..2148e8ce6e34 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1140,7 +1140,7 @@ void do_page_add_anon_rmap(struct page *page,
>  }
>  
>  /**
> - * page_add_new_anon_rmap - add pte mapping to a new anonymous page
> + * __page_add_new_anon_rmap - add pte mapping to a new anonymous page
>   * @page:	the page to add the mapping to
>   * @vma:	the vm area in which the mapping is added
>   * @address:	the user virtual address mapped
> @@ -1150,12 +1150,11 @@ void do_page_add_anon_rmap(struct page *page,
>   * This means the inc-and-test can be bypassed.
>   * Page does not have to be locked.
>   */
> -void page_add_new_anon_rmap(struct page *page,
> +void __page_add_new_anon_rmap(struct page *page,
>  	struct vm_area_struct *vma, unsigned long address, bool compound)
>  {
>  	int nr = compound ? hpage_nr_pages(page) : 1;
>  
> -	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
>  	__SetPageSwapBacked(page);
>  	if (compound) {
>  		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> -- 
> 2.21.0
>