[PATCH v2 3/4] block: Document the bio_vec properties

[Bart Van Assche <bvanassche@xxxxxxx>]

Since it is nontrivial that nth_page() does not have to be used for a
bio_vec, document this.

CC: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 include/linux/bvec.h | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index a81c13ac1972..ac0c7299d5b8 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -12,8 +12,17 @@
 #include <linux/errno.h>
 #include <linux/mm.h>
 
-/*
- * was unsigned short, but we might as well be ready for > 64kB I/O pages
+/**
+ * struct bio_vec - a contiguous range of physical memory addresses
+ * @bv_page:   First page associated with the address range.
+ * @bv_len:    Number of bytes in the address range.
+ * @bv_offset: Start of the address range relative to the start of @bv_page.
+ *
+ * The following holds for a bvec if n * PAGE_SIZE < bv_offset + bv_len:
+ *
+ *   nth_page(@bv_page, n) == @bv_page + n
+ *
+ * This holds because page_is_mergeable() checks the above property.
  */
 struct bio_vec {
 	struct page	*bv_page;