mm/memory.c:do_shared_fault(): add comment

Belatedly document the changes in commit f0c6d4d ("mm: introduce do_shared_fault() and drop do_fault()"). Cc: Andi Kleen <[email protected]> Cc: Bob Liu <[email protected]> Cc: Dave Hansen <[email protected]> Cc: "Kirill A. Shutemov" <[email protected]> Cc: Matthew Wilcox <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Naoya Horiguchi <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Sasha Levin <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
arunthomas · Dec 19, 2014 · d82fa87 · d82fa87
1 parent 48332f2
commit d82fa87
Showing 1 changed file with 6 additions and 0 deletions.
diff --git a/mm/memory.c b/mm/memory.c
@@ -2996,6 +2996,12 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (set_page_dirty(fault_page))
 		dirtied = 1;
+	/*
+	 * Take a local copy of the address_space - page.mapping may be zeroed
+	 * by truncate after unlock_page().   The address_space itself remains
+	 * pinned by vma->vm_file's reference.  We rely on unlock_page()'s
+	 * release semantics to prevent the compiler from undoing this copying.
+	 */
 	mapping = fault_page->mapping;
 	unlock_page(fault_page);
 	if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {