mm: use __SetPageSwapBacked and dont ClearPageSwapBacked

v3.16 commit 07a4278843 ("mm: shmem: avoid atomic operation during
shmem_getpage_gfp") rightly replaced one instance of SetPageSwapBacked
by __SetPageSwapBacked, pointing out that the newly allocated page is
not yet visible to other users (except speculative get_page_unless_zero-
ers, who may not update page flags before their further checks).

That was part of a series in which Mel was focused on tmpfs profiles:
but almost all SetPageSwapBacked uses can be so optimized, with the same
justification.

Remove ClearPageSwapBacked from __read_swap_cache_async() error path:
it's not an error to free a page with PG_swapbacked set.

Follow a convention of __SetPageLocked, __SetPageSwapBacked instead of
doing it differently in different places; but that's for tidiness - if
the ordering actually mattered, we should not be using the __variants.

There's probably scope for further __SetPageFlags in other places, but
SwapBacked is the one I'm interested in at the moment.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Yang Shi <yang.shi@linaro.org>
Cc: Ning Qu <quning@gmail.com>
Reviewed-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -1085,8 +1085,8 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
 	flush_dcache_page(newpage);
 
 	__SetPageLocked(newpage);
+	__SetPageSwapBacked(newpage);
 	SetPageUptodate(newpage);
-	SetPageSwapBacked(newpage);
 	set_page_private(newpage, swap_index);
 	SetPageSwapCache(newpage);
 
@@ -1276,8 +1276,8 @@ repeat:
 			goto decused;
 		}
 
-		__SetPageSwapBacked(page);
 		__SetPageLocked(page);
+		__SetPageSwapBacked(page);
 		if (sgp == SGP_WRITE)
 			__SetPageReferenced(page);