bio: don't overflow in bio_get_nr_vecs()

There were two places bio_get_nr_vecs() could overflow:

First, it did a left shift to convert from sectors to bytes immediately
before dividing by PAGE_SIZE.  If PAGE_SIZE ever was less than 512 a great
many things would break, so dividing by PAGE_SIZE >> 9 is safe and will
generate smaller code too.

The nastier overflow was in the DIV_ROUND_UP() (that's what the code was
effectively doing, anyways).  If n + d overflowed, the whole thing would
return 0 which breaks things rather effectively.

bio_get_nr_vecs() doesn't claim to give an exact value anyways, so the
DIV_ROUND_UP() is silly; we could do a straight divide except if a
device's queue_max_sectors was less than PAGE_SIZE we'd return 0.  So we
just add 1; this should always be safe - things will break badly if
bio_get_nr_vecs() returns > BIO_MAX_PAGES (bio_alloc() will suddenly start
failing) but it's queue_max_segments that must guard against this, if
queue_max_sectors is preventing this from happen things are going to
explode on architectures with different PAGE_SIZE.
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/bio.c

@@ -505,13 +505,9 @@ EXPORT_SYMBOL(bio_clone);
 int bio_get_nr_vecs(struct block_device *bdev)
 {
 	struct request_queue *q = bdev_get_queue(bdev);
-	int nr_pages;
-
-	nr_pages = ((queue_max_sectors(q) << 9) + PAGE_SIZE - 1) >> PAGE_SHIFT;
-	if (nr_pages > queue_max_segments(q))
-		nr_pages = queue_max_segments(q);
-
-	return nr_pages;
+	return min_t(unsigned,
+		     queue_max_segments(q),
+		     queue_max_sectors(q) / (PAGE_SIZE >> 9) + 1);
 }
 EXPORT_SYMBOL(bio_get_nr_vecs);