[PATCH BUGFIX V4] block: add missing group association in bio-cloning functions

[Paolo Valente]

When a bio is cloned, 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 cloning frequency, 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 broken.

This commit adds the missing association in bio-cloning functions.

Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
Reviewed-by: Nikolay Borisov <kernel@xxxxxxxx>
---
Tejun: I didn't add also your Ack, just because this version is slightly
different from the one you acked.
---
 block/bio.c          | 15 +++++++++++++++
 fs/btrfs/extent_io.c |  6 ------
 include/linux/bio.h  |  3 +++
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 807d25e..89f517c 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -590,6 +590,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
 	bio->bi_rw = bio_src->bi_rw;
 	bio->bi_iter = bio_src->bi_iter;
 	bio->bi_io_vec = bio_src->bi_io_vec;
+
+	bio_clone_blkcg_association(bio, bio_src);
 }
 EXPORT_SYMBOL(__bio_clone_fast);
 
@@ -695,6 +697,8 @@ integrity_clone:
 		}
 	}
 
+	bio_clone_blkcg_association(bio, bio_src);
+
 	return bio;
 }
 EXPORT_SYMBOL(bio_clone_bioset);
@@ -2016,6 +2020,17 @@ void bio_disassociate_task(struct bio *bio)
 	}
 }
 
+/**
+ * bio_clone_blkcg_association - clone blkcg association from src to dst bio
+ * @dst: destination bio
+ * @src: source bio
+ */
+void bio_clone_blkcg_association(struct bio *dst, struct bio *src)
+{
+	if (src->bi_css)
+		WARN_ON(bio_associate_blkcg(dst, src->bi_css));
+}
+
 #endif /* CONFIG_BLK_CGROUP */
 
 static void __init biovec_init_slabs(void)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index d247fc0..19f6739 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2686,12 +2686,6 @@ struct bio *btrfs_bio_clone(struct bio *bio, gfp_t gfp_mask)
 		btrfs_bio->csum = NULL;
 		btrfs_bio->csum_allocated = NULL;
 		btrfs_bio->end_io = NULL;
-
-#ifdef CONFIG_BLK_CGROUP
-		/* FIXME, put this into bio_clone_bioset */
-		if (bio->bi_css)
-			bio_associate_blkcg(new, bio->bi_css);
-#endif
 	}
 	return new;
 }
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 6b7481f..16cbe59 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -527,11 +527,14 @@ extern unsigned int bvec_nr_vecs(unsigned short idx);
 int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css);
 int bio_associate_current(struct bio *bio);
 void bio_disassociate_task(struct bio *bio);
+void bio_clone_blkcg_association(struct bio *dst, struct bio *src);
 #else	/* CONFIG_BLK_CGROUP */
 static inline int bio_associate_blkcg(struct bio *bio,
 			struct cgroup_subsys_state *blkcg_css) { return 0; }
 static inline int bio_associate_current(struct bio *bio) { return -ENOENT; }
 static inline void bio_disassociate_task(struct bio *bio) { }
+static inline void bio_clone_blkcg_association(struct bio *dst,
+			struct bio *src) { return 0; }
 #endif	/* CONFIG_BLK_CGROUP */
 
 #ifdef CONFIG_HIGHMEM
-- 
1.9.1