mm: fix PageUptodate data race

After running SetPageUptodate, preceeding stores to the page contents to
actually bring it uptodate may not be ordered with the store to set the
page uptodate.

Therefore, another CPU which checks PageUptodate is true, then reads the
page contents can get stale data.

Fix this by having an smp_wmb before SetPageUptodate, and smp_rmb after
PageUptodate.

Many places that test PageUptodate, do so with the page locked, and this
would be enough to ensure memory ordering in those places if
SetPageUptodate were only called while the page is locked. Unfortunately
that is not always the case for some filesystems, but it could be an idea
for the future.

Also bring the handling of anonymous page uptodateness in line with that of
file backed page management, by marking anon pages as uptodate when they
_are_ uptodate, rather than when our implementation requires that they be
marked as such. Doing allows us to get rid of the smp_wmb's in the page
copying functions, which were especially added for anonymous pages for an
analogous memory ordering problem. Both file and anonymous pages are
handled with the same barriers.

FAQ:
Q. Why not do this in flush_dcache_page?
A. Firstly, flush_dcache_page handles only one side (the smb side) of the
ordering protocol; we'd still need smp_rmb somewhere. Secondly, hiding away
memory barriers in a completely unrelated function is nasty; at least in the
PageUptodate macros, they are located together with (half) the operations
involved in the ordering. Thirdly, the smp_wmb is only required when first
bringing the page uptodate, wheras flush_dcache_page should be called each time
it is written to through the kernel mapping. It is logically the wrong place to
put it.

Q. Why does this increase my text size / reduce my performance / etc.
A. Because it is adding the necessary instructions to eliminate the data-race.

Q. Can it be improved?
A. Yes, eg. if you were to create a rule that all SetPageUptodate operations
run under the page lock, we could avoid the smp_rmb places where PageUptodate
is queried under the page lock. Requires audit of all filesystems and at least
some would need reworking. That's great you're interested, I'm eagerly awaiting
your patches.

Signed-off-by: Nick Piggin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/highmem.h

···
 	void *addr = kmap_atomic(page, KM_USER0);
 	clear_user_page(addr, vaddr, page);
 	kunmap_atomic(addr, KM_USER0);
-	/* Make sure this page is cleared on other CPU's too before using it */
-	smp_wmb();
 }
 
 #ifndef __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE
···
 	copy_user_page(vto, vfrom, vaddr, to);
 	kunmap_atomic(vfrom, KM_USER0);
 	kunmap_atomic(vto, KM_USER1);
-	/* Make sure this page is cleared on other CPU's too before using it */
-	smp_wmb();
 }
 
 #endif

include/linux/page-flags.h

···
 #define ClearPageReferenced(page)	clear_bit(PG_referenced, &(page)->flags)
 #define TestClearPageReferenced(page) test_and_clear_bit(PG_referenced, &(page)->flags)
 
-#define PageUptodate(page)	test_bit(PG_uptodate, &(page)->flags)
+static inline int PageUptodate(struct page *page)
+{
+	int ret = test_bit(PG_uptodate, &(page)->flags);
+
+	/*
+	 * Must ensure that the data we read out of the page is loaded
+	 * _after_ we've loaded page->flags to check for PageUptodate.
+	 * We can skip the barrier if the page is not uptodate, because
+	 * we wouldn't be reading anything from it.
+	 *
+	 * See SetPageUptodate() for the other side of the story.
+	 */
+	if (ret)
+		smp_rmb();
+
+	return ret;
+}
+
+static inline void __SetPageUptodate(struct page *page)
+{
+	smp_wmb();
+	__set_bit(PG_uptodate, &(page)->flags);
 #ifdef CONFIG_S390
+	page_clear_dirty(page);
+#endif
+}
+
 static inline void SetPageUptodate(struct page *page)
 {
+#ifdef CONFIG_S390
 	if (!test_and_set_bit(PG_uptodate, &page->flags))
 		page_clear_dirty(page);
-}
 #else
-#define SetPageUptodate(page)	set_bit(PG_uptodate, &(page)->flags)
+	/*
+	 * Memory barrier must be issued before setting the PG_uptodate bit,
+	 * so that all previous stores issued in order to bring the page
+	 * uptodate are actually visible before PageUptodate becomes true.
+	 *
+	 * s390 doesn't need an explicit smp_wmb here because the test and
+	 * set bit already provides full barriers.
+	 */
+	smp_wmb();
+	set_bit(PG_uptodate, &(page)->flags);
 #endif
+}
+
 #define ClearPageUptodate(page)	clear_bit(PG_uptodate, &(page)->flags)
 
 #define PageDirty(page)		test_bit(PG_dirty, &(page)->flags)

mm/hugetlb.c

···
 
 	spin_unlock(&mm->page_table_lock);
 	copy_huge_page(new_page, old_page, address, vma);
+	__SetPageUptodate(new_page);
 	spin_lock(&mm->page_table_lock);
 
 	ptep = huge_pte_offset(mm, address & HPAGE_MASK);
···
 			goto out;
 		}
 		clear_huge_page(page, address);
+		__SetPageUptodate(page);
 
 		if (vma->vm_flags & VM_SHARED) {
 			int err;

mm/memory.c

···
 			memset(kaddr, 0, PAGE_SIZE);
 		kunmap_atomic(kaddr, KM_USER0);
 		flush_dcache_page(dst);
-		return;
-
-	}
-	copy_user_highpage(dst, src, va, vma);
+	} else
+		copy_user_highpage(dst, src, va, vma);
 }
 
 /*
···
 	if (!new_page)
 		goto oom;
 	cow_user_page(new_page, old_page, address, vma);
+	__SetPageUptodate(new_page);
 
 	/*
 	 * Re-check the pte - we dropped the lock
···
 	page = alloc_zeroed_user_highpage_movable(vma, address);
 	if (!page)
 		goto oom;
+	__SetPageUptodate(page);
 
 	entry = mk_pte(page, vma->vm_page_prot);
 	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
···
 				goto out;
 			}
 			copy_user_highpage(page, vmf.page, address, vma);
+			__SetPageUptodate(page);
 		} else {
 			/*
 			 * If the page will be shareable, see if the backing

mm/page_io.c

···
 	int ret = 0;
 
 	BUG_ON(!PageLocked(page));
-	ClearPageUptodate(page);
+	BUG_ON(PageUptodate(page));
 	bio = get_swap_bio(GFP_KERNEL, page_private(page), page,
 				end_swap_bio_read);
 	if (bio == NULL) {

mm/swap_state.c

···
 	int err;
 
 	BUG_ON(!PageLocked(page));
+	BUG_ON(!PageUptodate(page));
 
 	for (;;) {
 		entry = get_swap_page();
···
 
 		switch (err) {
 		case 0:				/* Success */
-			SetPageUptodate(page);
 			SetPageDirty(page);
 			return 1;
 		case -EEXIST: