crypto: skcipher - Use scatterwalk (un)map interface for dst and src buffers

The skcipher walk API implementation avoids scatterwalk_map() for
mapping the source and destination buffers, and invokes kmap_atomic()
directly if the buffer in question is not in low memory (which can only
happen on 32-bit architectures). This avoids some overhead on 64-bit
architectures, and most notably, permits the skcipher code to run with
preemption enabled.

Now that scatterwalk_map() has been updated to use kmap_local(), none of
this is needed, so we can simply use scatterwalk_map/unmap instead.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/skcipher.c

@@ -42,38 +42,24 @@ struct skcipher_walk_buffer {
 
 static int skcipher_walk_next(struct skcipher_walk *walk);
 
-static inline void skcipher_unmap(struct scatter_walk *walk, void *vaddr)
-{
-	if (PageHighMem(scatterwalk_page(walk)))
-		kunmap_atomic(vaddr);
-}
-
-static inline void *skcipher_map(struct scatter_walk *walk)
-{
-	struct page *page = scatterwalk_page(walk);
-
-	return (PageHighMem(page) ? kmap_atomic(page) : page_address(page)) +
-	       offset_in_page(walk->offset);
-}
-
 static inline void skcipher_map_src(struct skcipher_walk *walk)
 {
-	walk->src.virt.addr = skcipher_map(&walk->in);
+	walk->src.virt.addr = scatterwalk_map(&walk->in);
 }
 
 static inline void skcipher_map_dst(struct skcipher_walk *walk)
 {
-	walk->dst.virt.addr = skcipher_map(&walk->out);
+	walk->dst.virt.addr = scatterwalk_map(&walk->out);
 }
 
 static inline void skcipher_unmap_src(struct skcipher_walk *walk)
 {
-	skcipher_unmap(&walk->in, walk->src.virt.addr);
+	scatterwalk_unmap(walk->src.virt.addr);
 }
 
 static inline void skcipher_unmap_dst(struct skcipher_walk *walk)
 {
-	skcipher_unmap(&walk->out, walk->dst.virt.addr);
+	scatterwalk_unmap(walk->dst.virt.addr);
 }
 
 static inline gfp_t skcipher_walk_gfp(struct skcipher_walk *walk)