[PATCH v4 13/20] mm: Introduce __page_add_new_anon_rmap()

[Laurent Dufour]

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@xxxxxxxxxxxxxxxxxx>
---
 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 733d3d8181e2..d91be69c1c60 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -173,8 +173,16 @@ 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 3ea1bbe6cb03..7cbf03f1a7ca 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2551,7 +2551,7 @@ static int 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);
 		/*
@@ -3065,7 +3065,7 @@ int 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 {
@@ -3215,7 +3215,7 @@ static int 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:
@@ -3467,7 +3467,7 @@ int 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 787c07fb37dc..357ea765e795 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1138,7 +1138,7 @@ 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
@@ -1148,12 +1148,11 @@ 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);
-- 
2.7.4