[PATCH BUGFIX] block: add missing group association in bio_split

[Paolo Valente]

When a bio is split, the newly created bio must be associated with the
same blkcg as the original bio (if BLK_CGROUP is enabled). If this
operation is not performed, then the new bio is not associated with
any group, and the group of the current task is returned when the
group of the bio is requested.

Depending on the frequency of splits, this may cause a large
percentage of the bios belonging to a given group to be treated as if
belonging to other groups (in most cases as if belonging to the root
group). The expected group isolation may thereby be then broken.

This commit adds the missing association in bio_split.

Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
---
 block/bio.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/bio.c b/block/bio.c
index 807d25e..c4a3834 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1811,6 +1811,11 @@ struct bio *bio_split(struct bio *bio, int sectors,
 
 	bio_advance(bio, split->bi_iter.bi_size);
 
+#ifdef CONFIG_BLK_CGROUP
+	if (bio->bi_css)
+		bio_associate_blkcg(split, bio->bi_css);
+#endif
+
 	return split;
 }
 EXPORT_SYMBOL(bio_split);
-- 
1.9.1