[PATCH 2/2] block: support adding less than len in bio_add_hw_page

[Christoph Hellwig <hch@lst.de>]

bio_add_hw_page currently always fails or succeeds.  This is fine for
the existing callers that always add PAGE_SIZE worth given that the
max_segment_size and max_sectors must always allow at least a page
worth of data.  But when we want to add it for bigger amounts of data
this means it can also fail when adding the data to a bio, and creating
a fallback for that becomes really annoying in the callers.

Make use of the existing API design that allows to return a smaller
length than the one passed in and add up to max_segment_size worth
of data from a larger input.  All the existing callers are fine with
this - not because they handle this return correctly, but because they
never pass more than a page in.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/bio.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index cef830adbc06e0..335d81398991b3 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -966,10 +966,13 @@ int bio_add_hw_page(struct request_queue *q, struct bio *bio,
 		struct page *page, unsigned int len, unsigned int offset,
 		unsigned int max_sectors, bool *same_page)
 {
+	unsigned int max_size = max_sectors << SECTOR_SHIFT;
+
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return 0;
 
-	if (((bio->bi_iter.bi_size + len) >> SECTOR_SHIFT) > max_sectors)
+	len = min3(len, max_size, queue_max_segment_size(q));
+	if (len > max_size - bio->bi_iter.bi_size)
 		return 0;
 
 	if (bio->bi_vcnt > 0) {
-- 
2.39.2