[AIO]: kiocb->private is too large for kiocb's on-stack.

sizeof(struct kiocb) is dangerously large for a structure commonly
allocated on-stack. This patch converts the 24*sizeof(long) field,
->private, to a void pointer for use by file_operations entrypoints.
A ->dtor() method is added to the kiocb in order to support the release
of dynamically allocated structures referred to by ->private.

The sole in-tree users of ->private are async network read/write,
which are not, in fact, async, and so need not handle preallocated
->private as they would need to if ->ki_retry were ever used. The sole
truly async operations are direct IO pread()/pwrite() which do not
now use ->ki_retry(). All they would need to do in that case is to
check for ->private already being allocated for async kiocbs.

This rips 88B off the stack on 32-bit in the common case.

Signed-off-by: William Lee Irwin III <wli@holomorphy.com>
Signed-off-by: David S. Miller <davem@redhat.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/include/linux/aio.h b/include/linux/aio.h
index 93fe78800740..461a3b0736e0 100644
--- a/include/linux/aio.h
+++ b/include/linux/aio.h
@@ -23,8 +23,6 @@ struct kioctx;
 
 #define KIOCB_SYNC_KEY		(~0U)
 
-#define KIOCB_PRIVATE_SIZE	(24 * sizeof(long))
-
 /* ki_flags bits */
 #define KIF_LOCKED		0
 #define KIF_KICKED		1
@@ -55,6 +53,7 @@ struct kiocb {
 	struct kioctx		*ki_ctx;	/* may be NULL for sync ops */
 	int			(*ki_cancel)(struct kiocb *, struct io_event *);
 	long			(*ki_retry)(struct kiocb *);
+	void			(*ki_dtor)(struct kiocb *);
 
 	struct list_head	ki_list;	/* the aio core uses this
 						 * for cancellation */
@@ -65,8 +64,7 @@ struct kiocb {
 	} ki_obj;
 	__u64			ki_user_data;	/* user's data for completion */
 	loff_t			ki_pos;
-
-	char			private[KIOCB_PRIVATE_SIZE];
+	void			*private;
 };
 
 #define is_sync_kiocb(iocb)	((iocb)->ki_key == KIOCB_SYNC_KEY)
@@ -79,6 +77,7 @@ struct kiocb {
 		(x)->ki_filp = (filp);			\
 		(x)->ki_ctx = &tsk->active_mm->default_kioctx;	\
 		(x)->ki_cancel = NULL;			\
+		(x)->ki_dtor = NULL;			\
 		(x)->ki_obj.tsk = tsk;			\
 	} while (0)