[PATCH v2 alt 2] fs/direct-io.c: don't try to allocate more than BIO_MAX_PAGES in a bio

[David Dillow <dillowda@ornl.gov>]

When using devices that support max_segments > BIO_MAX_PAGES (256),
direct IO tries to allocate a bio with more pages than allowed, which
leads to an oops in dio_bio_alloc(). Clamp the request to the supported
maximum, and change dio_bio_alloc() to reflect that bio_alloc() will
always return a bio when called with __GFP_WAIT and a valid number of
vectors.
    
Signed-off-by: David Dillow <dillowda@ornl.gov>
--
Second alternate, my preferred that reflects that bio_alloc() cannot
fail when properly called. I can take or leave the comment.


diff --git a/fs/direct-io.c b/fs/direct-io.c
index 85882f6..96a01d8 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -325,13 +325,17 @@ void dio_end_io(struct bio *bio, int error)
 }
 EXPORT_SYMBOL_GPL(dio_end_io);
 
-static int
+static void
 dio_bio_alloc(struct dio *dio, struct block_device *bdev,
 		sector_t first_sector, int nr_vecs)
 {
 	struct bio *bio;
 
+	/* bio_alloc() is guaranteed to return a bio when called with
+	 * __GFP_WAIT and we request a valid number of vectors.
+	 */
 	bio = bio_alloc(GFP_KERNEL, nr_vecs);
+	BUG_ON(!bio);
 
 	bio->bi_bdev = bdev;
 	bio->bi_sector = first_sector;
@@ -342,7 +346,6 @@ dio_bio_alloc(struct dio *dio, struct block_device *bdev,
 
 	dio->bio = bio;
 	dio->logical_offset_in_bio = dio->cur_page_fs_offset;
-	return 0;
 }
 
 /*
@@ -583,8 +586,9 @@ static int dio_new_bio(struct dio *dio, sector_t start_sector)
 		goto out;
 	sector = start_sector << (dio->blkbits - 9);
 	nr_pages = min(dio->pages_in_io, bio_get_nr_vecs(dio->map_bh.b_bdev));
+	nr_pages = min(nr_pages, BIO_MAX_PAGES);
 	BUG_ON(nr_pages <= 0);
-	ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
+	dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
 	dio->boundary = 0;
 out:
 	return ret;