shmem: skip page split if we're not reclaiming

In theory when info->flags & VM_LOCKED we should not be getting
shem_writepage() called so we should be verifying this with a
WARN_ON_ONCE().  Since we should not be swapping then best to ensure we
also don't do the folio split earlier too.  So just move the check early
to avoid folio splits in case its a dubious call.

We also have a similar early bail when !total_swap_pages so just move that
earlier to avoid the possible folio split in the same situation.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Luis Chamberlain <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Christian Brauner <[email protected]>
Reviewed-by: Yosry Ahmed <[email protected]>
Tested-by: Xin Hao <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Cc: Adam Manzanares <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Pankaj Raghav <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/shmem.c

@@ -1347,6 +1347,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	if (WARN_ON_ONCE(!wbc->for_reclaim))
 		goto redirty;
 
+	if (WARN_ON_ONCE(info->flags & VM_LOCKED))
+		goto redirty;
+
+	if (!total_swap_pages)
+		goto redirty;
+
 	/*
 	 * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or
 	 * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
@@ -1362,10 +1368,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	}
 
 	index = folio->index;
-	if (info->flags & VM_LOCKED)
-		goto redirty;
-	if (!total_swap_pages)
-		goto redirty;
 
 	/*
 	 * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC