[PATCH] bio_get_nr_vecs

Add bio_get_nr_vecs(). It returns an approximate number of pages that
can be added to a block device. It's just a ballpark number, but I think
this is quite fine for the type of thing it is needed for: mpage etc
need to know an approx size of a bio that they need to allocate. It
would be silly to continously allocate 64-page sized bio_vec entries, if
the target cannot do more than 8, for example.

fs/bio.c

@@ -321,6 +321,29 @@ struct bio *bio_copy(struct bio *bio, int gfp_mask, int copy)
 	return NULL;
 }
 
+/**
+ *	bio_get_nr_vecs		- return approx number of vecs
+ *	@bdev:  I/O target
+ *
+ *	Return the approximate number of pages we can send to this target.
+ *	There's no guarentee that you will be able to fit this number of pages
+ *	into a bio, it does not account for dynamic restrictions that vary
+ *	on offset.
+ */
+int bio_get_nr_vecs(struct block_device *bdev)
+{
+	request_queue_t *q = bdev_get_queue(bdev);
+	int nr_pages;
+
+	nr_pages = q->max_sectors >> (PAGE_SHIFT - 9);
+	if (nr_pages > q->max_phys_segments)
+		nr_pages = q->max_phys_segments;
+	if (nr_pages > q->max_hw_segments)
+		nr_pages = q->max_hw_segments;
+
+	return nr_pages;
+}
+
 /**
  *	bio_add_page	-	attempt to add page to bio
  *	@bio: destination bio
@@ -635,3 +658,4 @@ EXPORT_SYMBOL(bio_clone);
 EXPORT_SYMBOL(bio_phys_segments);
 EXPORT_SYMBOL(bio_hw_segments);
 EXPORT_SYMBOL(bio_add_page);
+EXPORT_SYMBOL(bio_get_nr_vecs);

include/linux/bio.h

@@ -206,6 +206,7 @@ extern struct bio *bio_copy(struct bio *, int, int);
 extern inline void bio_init(struct bio *);
 
 extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
+extern int bio_get_nr_vecs(struct block_device *);
 
 #ifdef CONFIG_HIGHMEM
 /*