iov_iter: Add extraction functions

Hi Al, Jeff,

Here's a replacement for the extract_iter_to_iter() patch I had previously.
It's a WIP, some bits aren't fully implemented, though some bits I have tested
and got to work, but if you could take a look and see if you're okay with the
interface.

I think I've addressed most of Al's comments.  The page-pinning is conditional
on certain types of iterator, and a number of the iterator types just extract
to the same thing.  It should now handle kvec-class iterators that refer to
vmalloc'd data.

I've also added extraction to scatterlist (which I'll need for doing various
crypto things) and extraction to ib_sge which could be used in cifs/smb RDMA,
bypassing the conversion-to-scatterlist step.

As mentioned, there are bits that aren't fully implemented, let alone tested.

David

include/linux/uio2.h

@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* iov_iter extractors
+ *
+ * Copyright (C) 2022 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#ifndef _LINUX_UIO2_H
+#define _LINUX_UIO2_H
+
+#include <linux/uio.h>
+#include <linux/dma-direction.h>
+
+struct sg_table;
+struct ib_device;
+struct ib_sge;
+
+/*
+ * Cleanup information for an extracted iterator.
+ */
+struct iov_iter_cleanup {
+	union {
+		struct bio_vec	*bv;
+		struct kvec	*kv;
+	};
+	unsigned int	nr_segs;
+	bool		pages_pinned;
+	enum iter_type	type:8;
+};
+
+ssize_t extract_iter_to_iter(struct iov_iter *iter, size_t len,
+			     struct iov_iter *to,
+			     struct iov_iter_cleanup *cleanup);
+ssize_t extract_iter_to_sg(struct iov_iter *iter, size_t len,
+			   struct sg_table *sgtable, bool *pinned);
+ssize_t extract_iter_to_rdma(struct iov_iter *iter, size_t len,
+			     struct ib_device *device, u32 local_dma_lkey,
+			     enum dma_data_direction direction,
+			     struct ib_sge *sge, unsigned int max_sge,
+			     unsigned int *nr_sge);
+void iov_iter_flush_dcache(struct iov_iter *iter,
+			   struct iov_iter_cleanup *cleanup);
+void iov_iter_clean_up(struct iov_iter_cleanup *cleanup);
+
+#endif /* _LINUX_UIO2_H */

lib/Makefile

@@ -44,7 +44,7 @@ obj-y	+= lockref.o
 
 obj-y += bcd.o sort.o parser.o debug_locks.o random32.o \
 	 bust_spinlocks.o kasprintf.o bitmap.o scatterlist.o \
-	 list_sort.o uuid.o iov_iter.o clz_ctz.o \
+	 list_sort.o uuid.o iov_iter.o iov_iter_extract.o clz_ctz.o \
 	 bsearch.o find_bit.o llist.o memweight.o kfifo.o \
 	 percpu-refcount.o rhashtable.o base64.o \
 	 once.o refcount.o usercopy.o errseq.o bucket_locks.o \