[PATCH V8 05/18] block: introduce bvec_last_segment()

[Ming Lei]

BTRFS and guard_bio_eod() need to get the last singlepage segment
from one multipage bvec, so introduce this helper to make them happy.

Cc: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
 include/linux/bvec.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 3d61352cd8cf..01616a0b6220 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -216,4 +216,29 @@ static inline bool mp_bvec_iter_advance(const struct bio_vec *bv,
 	.bi_bvec_done	= 0,						\
 }
 
+/*
+ * Get the last singlepage segment from the multipage bvec and store it
+ * in @seg
+ */
+static inline void bvec_last_segment(const struct bio_vec *bvec,
+		struct bio_vec *seg)
+{
+	unsigned total = bvec->bv_offset + bvec->bv_len;
+	unsigned last_page = total / PAGE_SIZE;
+
+	if (last_page * PAGE_SIZE == total)
+		last_page--;
+
+	seg->bv_page = nth_page(bvec->bv_page, last_page);
+
+	/* the whole segment is inside the last page */
+	if (bvec->bv_offset >= last_page * PAGE_SIZE) {
+		seg->bv_offset = bvec->bv_offset % PAGE_SIZE;
+		seg->bv_len = bvec->bv_len;
+	} else {
+		seg->bv_offset = 0;
+		seg->bv_len = total - last_page * PAGE_SIZE;
+	}
+}
+
 #endif /* __LINUX_BVEC_ITER_H */
-- 
2.9.5