[PATCH 2/8] mm: wp lock page before deciding cow

[Hugh Dickins <hugh@veritas.com>]

An application may rely on get_user_pages() to give it pages writable
from userspace and shared with a driver, GUP breaking COW if necessary.
It may mprotect() the pages' writability, off and on, from time to time.

Normally this works fine (so long as the app does not fork); but just
occasionally, under memory pressure, a readonly pte in a newly writable
area is COWed unnecessarily, breaking the link with the driver: because
do_wp_page() does trylock_page, and falls back to COW whenever that fails.

For reliable behaviour in the unshared case, when the trylock_page fails,
now unlock pagetable, lock page and relock pagetable, before deciding
whether Copy-On-Write is really necessary.

Reported-by: Zhou Yingchao
Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
This is not the patch I posted in June: in the end I decided it better just
to relock page as Nick suggested, than impose subtle ordering constraints
elsewhere; and also realized that page migration spoilt my optimizations.

 mm/memory.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

--- swapfree1/mm/memory.c	2008-11-21 18:50:41.000000000 +0000
+++ swapfree2/mm/memory.c	2008-11-21 18:50:43.000000000 +0000
@@ -1819,10 +1819,21 @@ static int do_wp_page(struct mm_struct *
 	 * not dirty accountable.
 	 */
 	if (PageAnon(old_page)) {
-		if (trylock_page(old_page)) {
-			reuse = can_share_swap_page(old_page);
-			unlock_page(old_page);
+		if (!trylock_page(old_page)) {
+			page_cache_get(old_page);
+			pte_unmap_unlock(page_table, ptl);
+			lock_page(old_page);
+			page_table = pte_offset_map_lock(mm, pmd, address,
+							 &ptl);
+			if (!pte_same(*page_table, orig_pte)) {
+				unlock_page(old_page);
+				page_cache_release(old_page);
+				goto unlock;
+			}
+			page_cache_release(old_page);
 		}
+		reuse = can_share_swap_page(old_page);
+		unlock_page(old_page);
 	} else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
 					(VM_WRITE|VM_SHARED))) {
 		/*

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