[PATCH 03/19] block: add a bio_add_max_vecs helper

Christoph Hellwig
2025-04-30T21:22:10Z

Add a helper to check how many bio_vecs are needed to add a kernel
virtual address range to a bio, accounting for the always contiguous
direct mapping and vmalloc mappings that usually need a bio_vec
per page sized chunk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
include/linux/bio.h | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index ad54e6af20dc..128b1c6ca648 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -418,6 +418,21 @@ void bio_add_folio_nofail(struct bio *bio, struct folio *folio, size_t len,
size_t off);
void bio_add_virt_nofail(struct bio *bio, void *vaddr, unsigned len);

+/**
+ * bio_add_max_vecs - number of bio_vecs needed to add data to a bio
+ * @kaddr: kernel virtual address to add
+ * @len: length in bytes to add
+ *
+ * Calculate how many bio_vecs need to be allocated to add the kernel virtual
+ * address range in [@kaddr:@len] in the worse case.
+ */
+static inline unsigned int bio_add_max_vecs(void *kaddr, unsigned int len)
+{
+ if (is_vmalloc_addr(kaddr))
+ return DIV_ROUND_UP(offset_in_page(kaddr) + len, PAGE_SIZE);
+ return 1;
+}
+
int submit_bio_wait(struct bio *bio);
int bdev_rw_virt(struct block_device *bdev, sector_t sector, void *data,
size_t len, enum req_op op);
--
2.47.2