[RFC PATCH 0/4] iov_iter: Add extraction helpers

[RFC PATCH 0/4] iov_iter: Add extraction helpers

[David Howells]

Hi Al,

Here are four patches to provide support for extracting pages from an
iov_iter, where such a thing makes sense, if you could take a look?

The first couple of patches provide iov_iter general stuff:

 (1) Move the FOLL_* flags to linux/mm_types.h so that linux/uio.h can make
     use of them.

 (2) 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 left untouched (on the assumption that
     the caller manages their lifetime).

     The determination is:

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

     It also adds a function by which the caller can determine which of
     "list only, get or pin" the extraction function will actually do 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:

 (3) Add a netfslib function to use (2) 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.

 (4) 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 (2) 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.

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/166697254399.61150.1256557652599252121.stgit@warthog.procyon.org.uk/
---
David Howells (4):
      mm: Move FOLL_* defs to mm_types.h
      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


 fs/netfs/Makefile        |   1 +
 fs/netfs/iterator.c      | 346 +++++++++++++++++++++++++++++++++++++++
 include/linux/mm.h       |  74 ---------
 include/linux/mm_types.h |  73 +++++++++
 include/linux/netfs.h    |   5 +
 include/linux/uio.h      |  29 ++++
 lib/iov_iter.c           | 333 +++++++++++++++++++++++++++++++++++++
 mm/vmalloc.c             |   1 +
 8 files changed, 788 insertions(+), 74 deletions(-)
 create mode 100644 fs/netfs/iterator.c

[RFC PATCH 3/4] 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, pinning the pages 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.  Pinning 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 pinned, so we want to pin
     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 assume that other types of iterator (eg. BVEC-, KVEC- and XARRAY-type)
are constructed only by kernel internals and that the pages are pinned in
those cases.

DISCARD- and PIPE-type iterators aren't DIO'able.

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
---

 fs/netfs/Makefile     |    1 +
 fs/netfs/iterator.c   |   94 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |    2 +
 3 files changed, 97 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..c11d05a66a4a
--- /dev/null
+++ b/fs/netfs/iterator.c
@@ -0,0 +1,94 @@
+// 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
+ *
+ * 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 and the original
+ * iterator will have been advanced by the amount extracted.
+ */
+ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
+				struct iov_iter *new)
+{
+	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;
+
+	/* 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, &offset);
+		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(netfs_extract_user_iter);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index f2402ddeafbf..5f6ad0246946 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -288,6 +288,8 @@ 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);
 
 /**
  * netfs_inode - Get the netfs inode context from the inode

[RFC PATCH 4/4] 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 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.

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
---

 fs/netfs/iterator.c   |  252 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |    3 +
 mm/vmalloc.c          |    1 
 3 files changed, 256 insertions(+)

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index c11d05a66a4a..62485416cc3d 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"
 
@@ -92,3 +94,253 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
 	return npages;
 }
 EXPORT_SYMBOL(netfs_extract_user_iter);
+
+/*
+ * Extract and pin up to sg_max pages from UBUF- or IOVEC-class iterators 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)
+{
+	struct scatterlist *sg = sgtable->sgl + sgtable->nents;
+	struct page **pages;
+	unsigned int npages;
+	ssize_t ret = 0, res;
+	size_t len, off;
+
+	/* 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_get_pages2(iter, pages, maxsize, sg_max, &off);
+		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)
+{
+	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);
+	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)
+{
+	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);
+	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)
+{
+	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);
+	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
+ *
+ * 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)
+{
+	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);
+	case ITER_BVEC:
+		return netfs_extract_bvec_to_sg(iter, maxsize, sgtable, sg_max);
+	case ITER_KVEC:
+		return netfs_extract_kvec_to_sg(iter, maxsize, sgtable, sg_max);
+	case ITER_XARRAY:
+		return netfs_extract_xarray_to_sg(iter, maxsize, sgtable, sg_max);
+	default:
+		pr_err("netfs_extract_iter_to_sg(%u) unsupported\n",
+		       iov_iter_type(iter));
+		WARN_ON_ONCE(1);
+		return -EIO;
+	}
+}
+EXPORT_SYMBOL(netfs_extract_iter_to_sg);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 5f6ad0246946..21771dd594a1 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -290,6 +290,9 @@ void netfs_put_subrequest(struct netfs_io_subrequest *subreq,
 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);
+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);
 
 /**
  * netfs_inode - Get the netfs inode context from the inode
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ccaa461998f3..b13ac142685b 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -653,6 +653,7 @@ 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

Re: [RFC PATCH 3/4] netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator

[David Howells]

I updated the commit message to stop using pinning in a general sense:

    netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator
    
    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 (WRITE) or pinning them (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).

David

Re: [RFC PATCH 4/4] netfs: Add a function to extract an iterator into a scatterlist

[David Howells]

I updated the patch description to mention that pinning may be used instead of
getting a ref:

    netfs: Add a function to extract an iterator into a scatterlist
    
    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 (WRITE) or a
    pin (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.

David