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-rw-r--r--fs/netfs/iterator.c103
1 files changed, 103 insertions, 0 deletions
diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
new file mode 100644
index 0000000000..2ff07ba655
--- /dev/null
+++ b/fs/netfs/iterator.c
@@ -0,0 +1,103 @@
+// 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/mm.h>
+#include <linux/uio.h>
+#include <linux/scatterlist.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
+ * @extraction_flags: Flags to qualify the request
+ *
+ * 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.
+ *
+ * @extraction_flags can have ITER_ALLOW_P2PDMA set to request peer-to-peer DMA be
+ * allowed on the pages extracted.
+ *
+ * On success, the number of elements in the bvec is returned, the original
+ * iterator will have been advanced by the amount extracted.
+ *
+ * The iov_iter_extract_mode() function should be used to query how cleanup
+ * should be performed.
+ */
+ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
+ struct iov_iter *new,
+ iov_iter_extraction_t extraction_flags)
+{
+ 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, extraction_flags,
+ &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;
+ bvec_set_page(bv + npages + i, *pages++, len - offset, offset);
+ ret -= len;
+ offset = 0;
+ }
+
+ npages += cur_npages;
+ }
+
+ iov_iter_bvec(new, orig->data_source, bv, npages, orig_len - count);
+ return npages;
+}
+EXPORT_SYMBOL_GPL(netfs_extract_user_iter);