[PATCH 11/16] 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. Currently __page_add_new_anon_rmap() is only
called during the speculative page fault path.

Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
---
 include/linux/rmap.h | 12 ++++++++++--
 mm/rmap.c            |  5 ++---
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 43ef2c30cb0f..f5cd4dbc78b0 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -170,8 +170,16 @@ 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/rmap.c b/mm/rmap.c
index c8993c63eb25..e99f9cd7b399 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1117,7 +1117,7 @@ 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
@@ -1127,12 +1127,11 @@ 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