io_uring: get rid of alloc cache init_once handling

init_once is called when an object doesn't come from the cache, and
hence needs initial clearing of certain members. While the whole
struct could get cleared by memset() in that case, a few of the cache
members are large enough that this may cause unnecessary overhead if
the caches used aren't large enough to satisfy the workload. For those
cases, some churn of kmalloc+kfree is to be expected.

Ensure that the 3 users that need clearing put the members they need
cleared at the start of the struct, and wrap the rest of the struct in
a struct group so the offset is known.

While at it, improve the interaction with KASAN such that when/if
KASAN writes to members inside the struct that should be retained over
caching, it won't trip over itself. For rw and net, the retaining of
the iovec over caching is disabled if KASAN is enabled. A helper will
free and clear those members in that case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rw.h

@@ -9,19 +9,24 @@ struct io_meta_state {
 
 struct io_async_rw {
 	size_t				bytes_done;
-	struct iov_iter			iter;
-	struct iov_iter_state		iter_state;
-	struct iovec			fast_iov;
 	struct iovec			*free_iovec;
-	int				free_iov_nr;
-	/* wpq is for buffered io, while meta fields are used with direct io */
-	union {
-		struct wait_page_queue		wpq;
-		struct {
-			struct uio_meta			meta;
-			struct io_meta_state		meta_state;
+	struct_group(clear,
+		struct iov_iter			iter;
+		struct iov_iter_state		iter_state;
+		struct iovec			fast_iov;
+		int				free_iov_nr;
+		/*
+		 * wpq is for buffered io, while meta fields are used with
+		 * direct io
+		 */
+		union {
+			struct wait_page_queue		wpq;
+			struct {
+				struct uio_meta			meta;
+				struct io_meta_state		meta_state;
+			};
 		};
-	};
+	);
 };
 
 int io_prep_read_fixed(struct io_kiocb *req, const struct io_uring_sqe *sqe);