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

[David Howells]

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 at redhat.com>
cc: Paulo Alcantara <pc at manguebit.org>
cc: Matthew Wilcox <willy at infradead.org>
cc: Christoph Hellwig <hch at infradead.org>
cc: Jens Axboe <axboe at kernel.dk>
cc: linux-block at vger.kernel.org
cc: netfs at lists.linux.dev
cc: linux-fsdevel at 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.
  *