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 <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Andres Lagar-Cavilla <[e-mail]>
Cc: Yang Shi <[e-mail]>
Cc: Ning Qu <[e-mail]>
Reviewed-by: Mel Gorman <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/migrate.c

···
 		newpage->index = page->index;
 		newpage->mapping = page->mapping;
 		if (PageSwapBacked(page))
-			SetPageSwapBacked(newpage);
+			__SetPageSwapBacked(newpage);
 
 		return MIGRATEPAGE_SUCCESS;
 	}
···
 	newpage->index = page->index;
 	newpage->mapping = page->mapping;
 	if (PageSwapBacked(page))
-		SetPageSwapBacked(newpage);
+		__SetPageSwapBacked(newpage);
 
 	get_page(newpage);	/* add cache reference */
 	if (PageSwapCache(page)) {
···
 
 	/* Prepare a page as a migration target */
 	__SetPageLocked(new_page);
-	SetPageSwapBacked(new_page);
+	__SetPageSwapBacked(new_page);
 
 	/* anon mapping, we can simply copy page->mapping to the new page: */
 	new_page->mapping = page->mapping;

mm/rmap.c

···
 	int nr = compound ? hpage_nr_pages(page) : 1;
 
 	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
-	SetPageSwapBacked(page);
+	__SetPageSwapBacked(page);
 	if (compound) {
 		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 		/* increment count (starts at -1) */

mm/shmem.c

···
 	flush_dcache_page(newpage);
 
 	__SetPageLocked(newpage);
+	__SetPageSwapBacked(newpage);
 	SetPageUptodate(newpage);
-	SetPageSwapBacked(newpage);
 	set_page_private(newpage, swap_index);
 	SetPageSwapCache(newpage);
 
···
 			goto decused;
 		}
 
-		__SetPageSwapBacked(page);
 		__SetPageLocked(page);
+		__SetPageSwapBacked(page);
 		if (sgp == SGP_WRITE)
 			__SetPageReferenced(page);
 

mm/swap_state.c

···
 
 		/* 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();
···
 			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