bvec: add a bvec_virt helper

Add a helper to get the virtual address for a bvec.  This avoids that
all callers need to know about the page + offset representation.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NChaitanya Kulkarni <kch@kernel.org>
Reviewed-by: NMartin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210804095634.460779-2-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>

include/linux/bvec.h

@@ -229,4 +229,16 @@ static inline void memzero_bvec(struct bio_vec *bvec)
 	memzero_page(bvec->bv_page, bvec->bv_offset, bvec->bv_len);
 }
 
+/**
+ * bvec_virt - return the virtual address for a bvec
+ * @bvec: bvec to return the virtual address for
+ *
+ * Note: the caller must ensure that @bvec->bv_page is not a highmem page.
+ */
+static inline void *bvec_virt(struct bio_vec *bvec)
+{
+	WARN_ON_ONCE(PageHighMem(bvec->bv_page));
+	return page_address(bvec->bv_page) + bvec->bv_offset;
+}
+
 #endif /* __LINUX_BVEC_H */