mm: use __SetPageSwapBacked and dont ClearPageSwapBacked

v3.16 commit 07a427884348 ("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>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 366ce3518703..0d457e7db8d6 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -358,7 +358,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 
                 /* May fail (-ENOMEM) if radix-tree node allocation failed. */
                 __SetPageLocked(new_page);
-                SetPageSwapBacked(new_page);
+                __SetPageSwapBacked(new_page);
                 err = __add_to_swap_cache(new_page, entry);
                 if (likely(!err)) {
                         radix_tree_preload_end();
@@ -370,7 +370,6 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                         return new_page;
                 }
                 radix_tree_preload_end();
-                ClearPageSwapBacked(new_page);
                 __ClearPageLocked(new_page);
                 /*
                  * add_to_swap_cache() doesn't return -EEXIST, so we can safely