btrfs: bioset allocations will never fail, adapt our helpers

Christoph pointed out that bio allocations backed by a bioset will never
fail.  As we always use a bioset for all bio allocations, we can skip
the error handling.  This patch adjusts our low-level helpers, the
cascaded changes to all callers will come next.

CC: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2659,8 +2659,9 @@ readpage_ok:
 }
 
 /*
- * this allocates from the btrfs_bioset.  We're returning a bio right now
- * but you can call btrfs_io_bio for the appropriate container_of magic
+ * The following helpers allocate a bio. As it's backed by a bioset, it'll
+ * never fail.  We're returning a bio right now but you can call btrfs_io_bio
+ * for the appropriate container_of magic
  */
 struct bio *
 btrfs_bio_alloc(struct block_device *bdev, u64 first_sector, int nr_vecs,
@@ -2670,22 +2671,12 @@ btrfs_bio_alloc(struct block_device *bdev, u64 first_sector, int nr_vecs,
 	struct bio *bio;
 
 	bio = bio_alloc_bioset(gfp_flags, nr_vecs, btrfs_bioset);
-
-	if (bio == NULL && (current->flags & PF_MEMALLOC)) {
-		while (!bio && (nr_vecs /= 2)) {
-			bio = bio_alloc_bioset(gfp_flags,
-					       nr_vecs, btrfs_bioset);
-		}
-	}
-
-	if (bio) {
 	bio->bi_bdev = bdev;
 	bio->bi_iter.bi_sector = first_sector;
 	btrfs_bio = btrfs_io_bio(bio);
 	btrfs_bio->csum = NULL;
 	btrfs_bio->csum_allocated = NULL;
 	btrfs_bio->end_io = NULL;
-	}
 	return bio;
 }
 
@@ -2694,30 +2685,27 @@ struct bio *btrfs_bio_clone(struct bio *bio, gfp_t gfp_mask)
 	struct btrfs_io_bio *btrfs_bio;
 	struct bio *new;
 
+	/* Bio allocation backed by a bioset does not fail */
 	new = bio_clone_fast(bio, gfp_mask, btrfs_bioset);
-	if (new) {
 	btrfs_bio = btrfs_io_bio(new);
 	btrfs_bio->csum = NULL;
 	btrfs_bio->csum_allocated = NULL;
 	btrfs_bio->end_io = NULL;
 	btrfs_bio->iter = bio->bi_iter;
-	}
 	return new;
 }
 
-/* this also allocates from the btrfs_bioset */
 struct bio *btrfs_io_bio_alloc(gfp_t gfp_mask, unsigned int nr_iovecs)
 {
 	struct btrfs_io_bio *btrfs_bio;
 	struct bio *bio;
 
+	/* Bio allocation backed by a bioset does not fail */
 	bio = bio_alloc_bioset(gfp_mask, nr_iovecs, btrfs_bioset);
-	if (bio) {
 	btrfs_bio = btrfs_io_bio(bio);
 	btrfs_bio->csum = NULL;
 	btrfs_bio->csum_allocated = NULL;
 	btrfs_bio->end_io = NULL;
-	}
 	return bio;
 }