[PATCH 14/26] netfs: Add a function to extract from an iter into a bvecq

David Howells
2026-03-26T10:48:16Z

Add a function to extract a slice of data from an iterator of any type into
a bvec queue chain.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>
cc: Steve French <sfrench@samba.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
fs/netfs/iterator.c | 123 ++++++++++++++++++++++++++++++++++++++++++
include/linux/netfs.h | 3 ++
2 files changed, 126 insertions(+)

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index adca78747f23..e77fd39327c2 100644
--- a/fs/netfs/iterator.c
+++ b/fs/netfs/iterator.c
@@ -13,6 +13,129 @@
#include <linux/netfs.h>
#include "internal.h"

+/**
+ * netfs_extract_iter - Extract the pages from an iterator into a bvecq
+ * @orig: The original iterator
+ * @orig_len: The amount of iterator to copy
+ * @max_segs: Maximum number of contiguous segments
+ * @fpos: Starting file position to label the bvecq with
+ * @_bvecq_head: Where to cache the bvec queue
+ * @extraction_flags: Flags to qualify the request
+ *
+ * Extract the page fragments from the given amount of the source iterator and
+ * build bvec queue 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 amount of data in the bvec is returned, the original
+ * iterator will have been advanced by the amount extracted.
+ *
+ * The bvecq segments are marked with indications on how to get clean up the
+ * extracted fragments.
+ */
+ssize_t netfs_extract_iter(struct iov_iter *orig, size_t orig_len, size_t max_segs,
+ unsigned long long fpos, struct bvecq **_bvecq_head,
+ iov_iter_extraction_t extraction_flags)
+{
+ struct bvecq *bq_tail = NULL;
+ ssize_t ret = 0;
+ size_t extracted = 0, nr_pages;
+
+ _enter("{%u,%zx},%zx", orig->iter_type, orig->count, orig_len);
+
+ WARN_ON_ONCE(orig_len > orig->count);
+
+ nr_pages = iov_iter_npages(orig, max_segs ?: INT_MAX);
+ if (WARN_ON(nr_pages == 0) ||
+ WARN_ON(nr_pages > max_segs))
+ nr_pages = max_segs;
+ max_segs = nr_pages;
+
+ do {
+ struct bvecq *bq;
+
+ if (WARN_ON(max_segs == 0))
+ break;
+
+ bq = bvecq_alloc_one(max_segs, GFP_NOFS);
+ if (!bq) {
+ ret = -ENOMEM;
+ break;
+ }
+ bq->free = user_backed_iter(orig);
+ bq->unpin = iov_iter_extract_will_pin(orig);
+ bq->prev = bq_tail;
+ bq->fpos = fpos + extracted;
+
+ if (bq_tail)
+ bq_tail->next = bq;
+ else
+ *_bvecq_head = bq;
+ bq_tail = bq;
+
+ if (orig_len == 0)
+ break;
+
+ struct bio_vec *bv = bq->bv;
+ do {
+ struct page **pages;
+ ssize_t got;
+ size_t offset;
+ size_t space = bq->max_slots - bq->nr_slots;
+ size_t bv_size = array_size(bq->max_slots, sizeof(*bv));
+ size_t pg_size = array_size(space, sizeof(*pages));
+
+ /* 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.
+ */
+ pages = (void *)bv + bv_size - pg_size;
+
+ got = iov_iter_extract_pages(orig, &pages, orig_len,
+ space, extraction_flags, &offset);
+ if (got < 0) {
+ ret = got;
+ goto out;
+ }
+
+ if (got == 0) {
+ pr_err("extract_pages gave nothing from %zu, %zu\n",
+ extracted, orig_len);
+ ret = -EIO;
+ goto out;
+ }
+
+ if (got > orig_len - extracted) {
+ pr_err("extract_pages rc=%zd more than %zu\n",
+ got, orig_len);
+ goto out;
+ }
+
+ extracted += got;
+ orig_len -= got;
+
+ do {
+ size_t len = umin(got, PAGE_SIZE - offset);
+
+ BUG_ON(bq->nr_slots >= bq->max_slots);
+
+ bvec_set_page(&bq->bv[bq->nr_slots],
+ *pages++, len, offset);
+ bq->nr_slots++;
+ got -= len;
+ offset = 0;
+ } while (got > 0);
+ } while (orig_len > 0 && !bvecq_is_full(bq));
+ } while (orig_len > 0 && max_segs > 0);
+
+out:
+ return extracted ?: ret;
+}
+EXPORT_SYMBOL_GPL(netfs_extract_iter);
+
/**
* netfs_extract_user_iter - Extract the pages from a user iterator into a bvec
* @orig: The original iterator
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 5bc48aacf7f6..b4602f7b6431 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -445,6 +445,9 @@ void netfs_get_subrequest(struct netfs_io_subrequest *subreq,
enum netfs_sreq_ref_trace what);
void netfs_put_subrequest(struct netfs_io_subrequest *subreq,
enum netfs_sreq_ref_trace what);
+ssize_t netfs_extract_iter(struct iov_iter *orig, size_t orig_len, size_t max_segs,
+ unsigned long long fpos, struct bvecq **_bvecq_head,
+ iov_iter_extraction_t extraction_flags);
ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
struct iov_iter *new,
iov_iter_extraction_t extraction_flags);