[RFC PATCH 04/17] Add a function to kmap one page of a multipage bio_vec

David Howells
2026-03-04T14:04:19Z

Add a function to kmap one page of a multipage bio_vec by offset (which is
added to the offset in the bio_vec internally). The caller is responsible
for calculating how much of the page is then available.

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: Jens Axboe <axboe@kernel.dk>
cc: linux-block@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
include/linux/bvec.h | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index d3c897270d40..01292021f51e 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -308,6 +308,27 @@ static inline phys_addr_t bvec_phys(const struct bio_vec *bvec)
return page_to_phys(bvec->bv_page) + bvec->bv_offset;
}

+/**
+ * kmap_local_bvec - Map part of a bvec into the kernel virtual address space
+ * @bvec: bvec to map
+ * @offset: Offset into bvec
+ *
+ * Map the page containing the byte at @offset into the kernel virtual address
+ * space. The caller is responsible for making sure this doesn't overrun.
+ *
+ * Call kunmap_local on the returned address to unmap.
+ */
+static inline void *kmap_local_bvec(struct bio_vec *bvec, size_t offset)
+{
+#ifdef CONFIG_HIGHMEM
+ offset += bvec->bv_offset;
+
+ return kmap_local_page(bvec->bv_page + offset / PAGE_SIZE) + offset % PAGE_SIZE;
+#else
+ return bvec_virt(bvec) + offset;
+#endif
+}
+
/*
* Segmented bio_vec queue.
*