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

Belatedly document the changes in commit f0c6d4d295e4 ("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]>

diff --git a/mm/memory.c b/mm/memory.c
index 6efe36a998bae484789441e3ea47e3b38b2ea346..d8aebc52265f59e75e342051fa604c5f1003f59b 100644 (file)
--- 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) {