[PATCH v4 0/7] iov_iter: Add extraction helpers

[PATCH v4 0/7] iov_iter: Add extraction helpers

[David Howells]

Hi Al,

Here are patches clean up some use of READ/WRITE and ITER_SOURCE/DEST,
patches to provide support for extracting pages from an iov_iter and a
patch to use the primary extraction function in the block layer bio code if
you could take a look?

[!] NOTE that I've switched the functions to be exported GPL-only at
    Christoph's request[1].  They are, however, intended as a replacement
    for iov_iter_get_pages*(), which is not marked _GPL - so that
    functionality will probably become unavailable to non-GPL 3rd party
    modules in future.

The first three patches deal with ITER_SOURCE/DEST:

 (1) Switch ITER_SOURCE/DEST to an enum and add a couple of helper
     functions to query if an iterator represents a source or a destination
     buffer.  Using an enum may allow extra consistency warnings from the
     compiler.

 (2) Use the ITER_SOURCE/DEST values in the iov_iter core functions rather
     than READ/WRITE.

 (3) Get rid of most of the callers of iov_iter_rw(), using the IOCB_WRITE
     and IOMAP_WRITE instead where available.  This leaves only two places
     looking at the this value: a consistency check in cifs and a single
     place in the block layer.

The next patch adds a replacement for iov_iter_get_pages*(), including
Logan's new version:

 (4) Add a function to list-only, get or pin pages from an iterator as a
     future replacement for iov_iter_get_pages*().  Pointers to the pages
     are placed into an array (which will get allocated if not provided)
     and, depending on the iterator type and direction, the pages will have
     a ref or a pin get on them or be left untouched (on the assumption
     that the caller manages their lifetime).

     The determination is:

	UBUF/IOVEC + DEST	-> pin
	UBUF/IOVEC + SOURCE	-> get
	PIPE + DEST		-> list-only
	BVEC/XARRAY		-> list-only
	Anything else		-> EFAULT

     The function also returns an indication of which of "list only, get or
     pin" the extraction function did to aid in cleaning up (returning 0,
     FOLL_GET or FOLL_PIN as appropriate).

Then there are a couple of patches that add stuff to netfslib that I want
to use there as well as in cifs:

 (5) Add a netfslib function to use (4) to extract pages from an ITER_IOBUF
     or ITER_UBUF iterator into an ITER_BVEC iterator.  This will get or
     pin the pages as appropriate.

 (6) Add a netfslib function to extract pages from an iterator that's of
     type ITER_UBUF/IOVEC/BVEC/KVEC/XARRAY and add them to a scatterlist.

     The function in (4) is used for a UBUF and IOVEC iterators, so those
     need cleaning up afterwards.  BVEC and XARRAY iterators are ungot and
     unpinned and may be rendered into elements that span multiple pages,
     for example if large folios are present.

Finally, there's an example test patch for the extract pages:

 (7) Make the block layer's BIO code pin pages or leave pages unaltered
     rather than getting a ref on the pages when the circumstances warrant,
     and noting in the bio struct what cleanups should be performed so that
     the bio cleanup code then does the right thing.

Changes:
========
ver #4)
 - Drop the patch to move the FOLL_* flags to linux/mm_types.h as they're
   no longer referenced by linux/uio.h.
 - Add patches 1-3 and 7.
 - Make patches 4-6 use ITER_SOURCE/DEST.
 - Allow additional gup_flags to be passed into iov_iter_extract_pages().

ver #3)
 - Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access
   to get/pin_user_pages_fast()[1].

ver #2)
 - Rolled the extraction cleanup mode query function into the extraction
   function, returning the indication through the argument list.
 - Fixed patch 4 (extract to scatterlist) to actually use the new
   extraction API.

I've pushed the patches here also:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=iov-extract

David

Link: https://lore.kernel.org/r/Y3zFzdWnWlEJ8X8/@infradead.org/ [1]
Link: https://lore.kernel.org/r/166697254399.61150.1256557652599252121.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166722777223.2555743.162508599131141451.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732024173.3186319.18204305072070871546.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166869687556.3723671.10061142538708346995.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166920902005.1461876.2786264600108839814.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/166997419665.9475.15014699817597102032.stgit@warthog.procyon.org.uk/ # v3

---
David Howells (7):
      iov_iter: Change the direction macros into an enum
      iov_iter: Use the direction in the iterator functions
      iov_iter: Use IOCB/IOMAP_WRITE if available rather than iterator direction
      iov_iter: Add a function to extract a page list from an iterator
      netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator
      netfs: Add a function to extract an iterator into a scatterlist
      iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate


 block/bio.c               |  47 +++--
 block/fops.c              |   8 +-
 fs/9p/vfs_addr.c          |   2 +-
 fs/affs/file.c            |   4 +-
 fs/ceph/file.c            |   2 +-
 fs/dax.c                  |   6 +-
 fs/direct-io.c            |  22 +--
 fs/exfat/inode.c          |   6 +-
 fs/ext2/inode.c           |   2 +-
 fs/f2fs/file.c            |  10 +-
 fs/fat/inode.c            |   4 +-
 fs/fuse/dax.c             |   2 +-
 fs/fuse/file.c            |   8 +-
 fs/hfs/inode.c            |   2 +-
 fs/hfsplus/inode.c        |   2 +-
 fs/iomap/direct-io.c      |   6 +-
 fs/jfs/inode.c            |   2 +-
 fs/netfs/Makefile         |   1 +
 fs/netfs/iterator.c       | 367 ++++++++++++++++++++++++++++++++++
 fs/nfs/direct.c           |   2 +-
 fs/nilfs2/inode.c         |   2 +-
 fs/ntfs3/inode.c          |   2 +-
 fs/ocfs2/aops.c           |   2 +-
 fs/orangefs/inode.c       |   2 +-
 fs/reiserfs/inode.c       |   2 +-
 fs/udf/inode.c            |   2 +-
 include/linux/blk_types.h |   1 +
 include/linux/netfs.h     |   7 +
 include/linux/uio.h       |  59 ++++--
 lib/iov_iter.c            | 407 +++++++++++++++++++++++++++++++++++---
 mm/vmalloc.c              |   1 +
 31 files changed, 889 insertions(+), 103 deletions(-)
 create mode 100644 fs/netfs/iterator.c

[PATCH v4 5/7] netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator

[David Howells]

Add a function to extract the pages from a user-space supplied iterator
(UBUF- or IOVEC-type) into a BVEC-type iterator, retaining the pages by
getting a ref on them (ITER_SOURCE, ie. WRITE) or pinning them (ITER_DEST,
ie. READ) as we go.

This is useful in three situations:

 (1) A userspace thread may have a sibling that unmaps or remaps the
     process's VM during the operation, changing the assignment of the
     pages and potentially causing an error.  Retaining the pages keeps
     some pages around, even if this occurs; futher, we find out at the
     point of extraction if EFAULT is going to be incurred.

 (2) Pages might get swapped out/discarded if not retained, so we want to
     retain them to avoid the reload causing a deadlock due to a DIO
     from/to an mmapped region on the same file.

 (3) The iterator may get passed to sendmsg() by the filesystem.  If a
     fault occurs, we may get a short write to a TCP stream that's then
     tricky to recover from.

We don't deal with other types of iterator here, leaving it to other
mechanisms to retain the pages (eg. PG_locked, PG_writeback and the pipe
lock).

Changes:
========
ver #3)
 - Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access
   to get/pin_user_pages_fast()[1].

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: linux-cachefs@redhat.com
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org

Link: https://lore.kernel.org/r/Y3zFzdWnWlEJ8X8/@infradead.org/ [1]
Link: https://lore.kernel.org/r/166697255265.61150.6289490555867717077.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732026503.3186319.12020462741051772825.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166869690376.3723671.8813331570219190705.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166920904810.1461876.11603559311247187100.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/166997422579.9475.12101700945635692496.stgit@warthog.procyon.org.uk/ # v3
---

 fs/netfs/Makefile     |    1 
 fs/netfs/iterator.c   |   99 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |    3 +
 3 files changed, 103 insertions(+)
 create mode 100644 fs/netfs/iterator.c

diff --git a/fs/netfs/Makefile b/fs/netfs/Makefile
index f684c0cd1ec5..386d6fb92793 100644
--- a/fs/netfs/Makefile
+++ b/fs/netfs/Makefile
@@ -3,6 +3,7 @@
 netfs-y := \
 	buffered_read.o \
 	io.o \
+	iterator.o \
 	main.o \
 	objects.o
 
diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
new file mode 100644
index 000000000000..7d802d21b9c5
--- /dev/null
+++ b/fs/netfs/iterator.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Iterator helpers.
+ *
+ * Copyright (C) 2022 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/export.h>
+#include <linux/slab.h>
+#include <linux/uio.h>
+#include <linux/netfs.h>
+#include "internal.h"
+
+/**
+ * netfs_extract_user_iter - Extract the pages from a user iterator into a bvec
+ * @orig: The original iterator
+ * @orig_len: The amount of iterator to copy
+ * @new: The iterator to be set up
+ * @cleanup_mode: Where to indicate the cleanup mode
+ *
+ * Extract the page fragments from the given amount of the source iterator and
+ * build up a second iterator that refers to all of those bits.  This allows
+ * the original iterator to disposed of.
+ *
+ * On success, the number of elements in the bvec is returned, the original
+ * iterator will have been advanced by the amount extracted and @*cleanup_mode
+ * will have been set to FOLL_GET, FOLL_PIN or 0.
+ */
+ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
+				struct iov_iter *new, unsigned int *cleanup_mode)
+{
+	struct bio_vec *bv = NULL;
+	struct page **pages;
+	unsigned int cur_npages;
+	unsigned int max_pages;
+	unsigned int npages = 0;
+	unsigned int i;
+	ssize_t ret;
+	size_t count = orig_len, offset, len;
+	size_t bv_size, pg_size;
+
+	if (WARN_ON_ONCE(!iter_is_ubuf(orig) && !iter_is_iovec(orig)))
+		return -EIO;
+
+	max_pages = iov_iter_npages(orig, INT_MAX);
+	bv_size = array_size(max_pages, sizeof(*bv));
+	bv = kvmalloc(bv_size, GFP_KERNEL);
+	if (!bv)
+		return -ENOMEM;
+
+	*cleanup_mode = 0;
+
+	/* Put the page list at the end of the bvec list storage.  bvec
+	 * elements are larger than page pointers, so as long as we work
+	 * 0->last, we should be fine.
+	 */
+	pg_size = array_size(max_pages, sizeof(*pages));
+	pages = (void *)bv + bv_size - pg_size;
+
+	while (count && npages < max_pages) {
+		ret = iov_iter_extract_pages(orig, &pages, count,
+					     max_pages - npages, 0,
+					     &offset, cleanup_mode);
+		if (ret < 0) {
+			pr_err("Couldn't get user pages (rc=%zd)\n", ret);
+			break;
+		}
+
+		if (ret > count) {
+			pr_err("get_pages rc=%zd more than %zu\n", ret, count);
+			break;
+		}
+
+		count -= ret;
+		ret += offset;
+		cur_npages = DIV_ROUND_UP(ret, PAGE_SIZE);
+
+		if (npages + cur_npages > max_pages) {
+			pr_err("Out of bvec array capacity (%u vs %u)\n",
+			       npages + cur_npages, max_pages);
+			break;
+		}
+
+		for (i = 0; i < cur_npages; i++) {
+			len = ret > PAGE_SIZE ? PAGE_SIZE : ret;
+			bv[npages + i].bv_page	 = *pages++;
+			bv[npages + i].bv_offset = offset;
+			bv[npages + i].bv_len	 = len - offset;
+			ret -= len;
+			offset = 0;
+		}
+
+		npages += cur_npages;
+	}
+
+	iov_iter_bvec(new, iov_iter_rw(orig), bv, npages, orig_len - count);
+	return npages;
+}
+EXPORT_SYMBOL_GPL(netfs_extract_user_iter);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 4c76ddfb6a67..26fe3e6bafa1 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -296,6 +296,9 @@ void netfs_get_subrequest(struct netfs_io_subrequest *subreq,
 void netfs_put_subrequest(struct netfs_io_subrequest *subreq,
 			  bool was_async, enum netfs_sreq_ref_trace what);
 void netfs_stats_show(struct seq_file *);
+ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
+				struct iov_iter *new,
+				unsigned int *cleanup_mode);
 
 /**
  * netfs_inode - Get the netfs inode context from the inode

[PATCH v4 6/7] netfs: Add a function to extract an iterator into a scatterlist

[David Howells]

Provide a function for filling in a scatterlist from the list of pages
contained in an iterator.

If the iterator is UBUF- or IOBUF-type, the pages have a ref (ITER_SOURCE,
ie. WRITE) or a pin (ITER_DEST, ie. READ) taken on them.

If the iterator is BVEC-, KVEC- or XARRAY-type, no ref is taken on the
pages and it is left to the caller to manage their lifetime.  It cannot be
assumed that a ref can be validly taken, particularly in the case of a KVEC
iterator.

Changes:
========
ver #3)
 - Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access
   to get/pin_user_pages_fast()[1].

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: linux-cachefs@redhat.com
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org

Link: https://lore.kernel.org/r/Y3zFzdWnWlEJ8X8/@infradead.org/ [1]
Link: https://lore.kernel.org/r/166697255985.61150.16489950598033809487.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732027275.3186319.5186488812166611598.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166869691313.3723671.10714823767342163891.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166920905749.1461876.12079195122363691498.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/166997423514.9475.11145024341505464337.stgit@warthog.procyon.org.uk/ # v3
---

 fs/netfs/iterator.c   |  268 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |    4 +
 mm/vmalloc.c          |    1 
 3 files changed, 273 insertions(+)

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index 7d802d21b9c5..82c46e3caf1d 100644
--- a/fs/netfs/iterator.c
+++ b/fs/netfs/iterator.c
@@ -7,7 +7,9 @@
 
 #include <linux/export.h>
 #include <linux/slab.h>
+#include <linux/mm.h>
 #include <linux/uio.h>
+#include <linux/scatterlist.h>
 #include <linux/netfs.h>
 #include "internal.h"
 
@@ -97,3 +99,269 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
 	return npages;
 }
 EXPORT_SYMBOL_GPL(netfs_extract_user_iter);
+
+/*
+ * Extract as list of up to sg_max pages from UBUF- or IOVEC-class iterators,
+ * pin or get refs on them appropriate and add them to the scatterlist.
+ */
+static ssize_t netfs_extract_user_to_sg(struct iov_iter *iter,
+					ssize_t maxsize,
+					struct sg_table *sgtable,
+					unsigned int sg_max,
+					unsigned int *cleanup_mode)
+{
+	struct scatterlist *sg = sgtable->sgl + sgtable->nents;
+	struct page **pages;
+	unsigned int npages;
+	ssize_t ret = 0, res;
+	size_t len, off;
+
+	*cleanup_mode = 0;
+
+	/* We decant the page list into the tail of the scatterlist */
+	pages = (void *)sgtable->sgl + array_size(sg_max, sizeof(struct scatterlist));
+	pages -= sg_max;
+
+	do {
+		res = iov_iter_extract_pages(iter, &pages, maxsize, sg_max, 0,
+					     &off, cleanup_mode);
+		if (res < 0)
+			goto failed;
+
+		len = res;
+		maxsize -= len;
+		ret += len;
+		npages = DIV_ROUND_UP(off + len, PAGE_SIZE);
+		sg_max -= npages;
+
+		for (; npages < 0; npages--) {
+			struct page *page = *pages;
+			size_t seg = min_t(size_t, PAGE_SIZE - off, len);
+
+			*pages++ = NULL;
+			sg_set_page(sg, page, len, off);
+			sgtable->nents++;
+			sg++;
+			len -= seg;
+			off = 0;
+		}
+	} while (maxsize > 0 && sg_max > 0);
+
+	return ret;
+
+failed:
+	while (sgtable->nents > sgtable->orig_nents)
+		put_page(sg_page(&sgtable->sgl[--sgtable->nents]));
+	return res;
+}
+
+/*
+ * Extract up to sg_max pages from a BVEC-type iterator and add them to the
+ * scatterlist.  The pages are not pinned.
+ */
+static ssize_t netfs_extract_bvec_to_sg(struct iov_iter *iter,
+					ssize_t maxsize,
+					struct sg_table *sgtable,
+					unsigned int sg_max,
+					unsigned int *cleanup_mode)
+{
+	const struct bio_vec *bv = iter->bvec;
+	struct scatterlist *sg = sgtable->sgl + sgtable->nents;
+	unsigned long start = iter->iov_offset;
+	unsigned int i;
+	ssize_t ret = 0;
+
+	for (i = 0; i < iter->nr_segs; i++) {
+		size_t off, len;
+
+		len = bv[i].bv_len;
+		if (start >= len) {
+			start -= len;
+			continue;
+		}
+
+		len = min_t(size_t, maxsize, len - start);
+		off = bv[i].bv_offset + start;
+
+		sg_set_page(sg, bv[i].bv_page, len, off);
+		sgtable->nents++;
+		sg++;
+		sg_max--;
+
+		ret += len;
+		maxsize -= len;
+		if (maxsize <= 0 || sg_max == 0)
+			break;
+		start = 0;
+	}
+
+	if (ret > 0)
+		iov_iter_advance(iter, ret);
+	*cleanup_mode = 0;
+	return ret;
+}
+
+/*
+ * Extract up to sg_max pages from a KVEC-type iterator and add them to the
+ * scatterlist.  This can deal with vmalloc'd buffers as well as kmalloc'd or
+ * static buffers.  The pages are not pinned.
+ */
+static ssize_t netfs_extract_kvec_to_sg(struct iov_iter *iter,
+					ssize_t maxsize,
+					struct sg_table *sgtable,
+					unsigned int sg_max,
+					unsigned int *cleanup_mode)
+{
+	const struct kvec *kv = iter->kvec;
+	struct scatterlist *sg = sgtable->sgl + sgtable->nents;
+	unsigned long start = iter->iov_offset;
+	unsigned int i;
+	ssize_t ret = 0;
+
+	for (i = 0; i < iter->nr_segs; i++) {
+		struct page *page;
+		unsigned long kaddr;
+		size_t off, len, seg;
+
+		len = kv[i].iov_len;
+		if (start >= len) {
+			start -= len;
+			continue;
+		}
+
+		kaddr = (unsigned long)kv[i].iov_base + start;
+		off = kaddr & ~PAGE_MASK;
+		len = min_t(size_t, maxsize, len - start);
+		kaddr &= PAGE_MASK;
+
+		maxsize -= len;
+		ret += len;
+		do {
+			seg = min_t(size_t, len, PAGE_SIZE - off);
+			if (is_vmalloc_or_module_addr((void *)kaddr))
+				page = vmalloc_to_page((void *)kaddr);
+			else
+				page = virt_to_page(kaddr);
+
+			sg_set_page(sg, page, len, off);
+			sgtable->nents++;
+			sg++;
+			sg_max--;
+
+			len -= seg;
+			kaddr += PAGE_SIZE;
+			off = 0;
+		} while (len > 0 && sg_max > 0);
+
+		if (maxsize <= 0 || sg_max == 0)
+			break;
+		start = 0;
+	}
+
+	if (ret > 0)
+		iov_iter_advance(iter, ret);
+	*cleanup_mode = 0;
+	return ret;
+}
+
+/*
+ * Extract up to sg_max folios from an XARRAY-type iterator and add them to
+ * the scatterlist.  The pages are not pinned.
+ */
+static ssize_t netfs_extract_xarray_to_sg(struct iov_iter *iter,
+					  ssize_t maxsize,
+					  struct sg_table *sgtable,
+					  unsigned int sg_max,
+					  unsigned int *cleanup_mode)
+{
+	struct scatterlist *sg = sgtable->sgl + sgtable->nents;
+	struct xarray *xa = iter->xarray;
+	struct folio *folio;
+	loff_t start = iter->xarray_start + iter->iov_offset;
+	pgoff_t index = start / PAGE_SIZE;
+	ssize_t ret = 0;
+	size_t offset, len;
+	XA_STATE(xas, xa, index);
+
+	rcu_read_lock();
+
+	xas_for_each(&xas, folio, ULONG_MAX) {
+		if (xas_retry(&xas, folio))
+			continue;
+		if (WARN_ON(xa_is_value(folio)))
+			break;
+		if (WARN_ON(folio_test_hugetlb(folio)))
+			break;
+
+		offset = offset_in_folio(folio, start);
+		len = min_t(size_t, maxsize, folio_size(folio) - offset);
+
+		sg_set_page(sg, folio_page(folio, 0), len, offset);
+		sgtable->nents++;
+		sg++;
+		sg_max--;
+
+		maxsize -= len;
+		ret += len;
+		if (maxsize <= 0 || sg_max == 0)
+			break;
+	}
+
+	rcu_read_unlock();
+	if (ret > 0)
+		iov_iter_advance(iter, ret);
+	*cleanup_mode = 0;
+	return ret;
+}
+
+/**
+ * netfs_extract_iter_to_sg - Extract pages from an iterator and add ot an sglist
+ * @iter: The iterator to extract from
+ * @maxsize: The amount of iterator to copy
+ * @sgtable: The scatterlist table to fill in
+ * @sg_max: Maximum number of elements in @sgtable that may be filled
+ * @cleanup_mode: Where to return the cleanup mode
+ *
+ * Extract the page fragments from the given amount of the source iterator and
+ * add them to a scatterlist that refers to all of those bits, to a maximum
+ * addition of @sg_max elements.
+ *
+ * The pages referred to by UBUF- and IOVEC-type iterators are extracted and
+ * pinned; BVEC-, KVEC- and XARRAY-type are extracted but aren't pinned; PIPE-
+ * and DISCARD-type are not supported.
+ *
+ * No end mark is placed on the scatterlist; that's left to the caller.
+ *
+ * If successul, @sgtable->nents is updated to include the number of elements
+ * added and the number of bytes added is returned.  @sgtable->orig_nents is
+ * left unaltered.
+ */
+ssize_t netfs_extract_iter_to_sg(struct iov_iter *iter, size_t maxsize,
+				 struct sg_table *sgtable, unsigned int sg_max,
+				 unsigned int *cleanup_mode)
+{
+	if (maxsize == 0)
+		return 0;
+
+	switch (iov_iter_type(iter)) {
+	case ITER_UBUF:
+	case ITER_IOVEC:
+		return netfs_extract_user_to_sg(iter, maxsize, sgtable, sg_max,
+						cleanup_mode);
+	case ITER_BVEC:
+		return netfs_extract_bvec_to_sg(iter, maxsize, sgtable, sg_max,
+						cleanup_mode);
+	case ITER_KVEC:
+		return netfs_extract_kvec_to_sg(iter, maxsize, sgtable, sg_max,
+						cleanup_mode);
+	case ITER_XARRAY:
+		return netfs_extract_xarray_to_sg(iter, maxsize, sgtable, sg_max,
+						  cleanup_mode);
+	default:
+		pr_err("netfs_extract_iter_to_sg(%u) unsupported\n",
+		       iov_iter_type(iter));
+		WARN_ON_ONCE(1);
+		return -EIO;
+	}
+}
+EXPORT_SYMBOL_GPL(netfs_extract_iter_to_sg);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 26fe3e6bafa1..059e49233e29 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -299,6 +299,10 @@ void netfs_stats_show(struct seq_file *);
 ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
 				struct iov_iter *new,
 				unsigned int *cleanup_mode);
+struct sg_table;
+ssize_t netfs_extract_iter_to_sg(struct iov_iter *iter, size_t len,
+				 struct sg_table *sgtable, unsigned int sg_max,
+				 unsigned int *cleanup_mode);
 
 /**
  * netfs_inode - Get the netfs inode context from the inode
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ca71de7c9d77..61f5bec0f2b6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -656,6 +656,7 @@ int is_vmalloc_or_module_addr(const void *x)
 #endif
 	return is_vmalloc_addr(x);
 }
+EXPORT_SYMBOL_GPL(is_vmalloc_or_module_addr);
 
 /*
  * Walk a vmap address to the struct page it maps. Huge vmap mappings will