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

[Paolo]

Il 10/05/2016 18:12, Jeff Moyer ha scritto:
> Paolo Valente <paolo.valente@xxxxxxxxxx> writes:
>
> > 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         | 15 +++++++++++++++
> >   include/linux/bio.h |  2 ++
> >   2 files changed, 17 insertions(+)
> >
> > diff --git a/block/bio.c b/block/bio.c
> > index 807d25e..20795eb 100644
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -1811,6 +1811,8 @@ struct bio *bio_split(struct bio *bio, int sectors,
> >
> >   	bio_advance(bio, split->bi_iter.bi_size);
> >
> > +	bio_clone_blkcg_association(split, bio);
> > +
>
> First, I think this should be done inside of bio_clone_fast and
> bio_clone_bioset instead of in bio_split.  Otherwise you miss other
> places where bios are cloned, and I'm guessing that's bad.  You could
> also then get rid of this code in btrfs/extent_io.c:
>
> #ifdef CONFIG_BLK_CGROUP
>                  /* FIXME, put this into bio_clone_bioset */
>                  if (bio->bi_css)
>                          bio_associate_blkcg(new, bio->bi_css);
> #endif
>
> Next, you went to the trouble of propagating the return value from
> bio_associate_blkcg, and then it goes unchecked here in the only caller
> of your new function.  Since bio_associate_blkcg should not fail when
> cloning (right?), I'd change bio_clone_blkcg_association to return void
> and to WARN_ON a failure return from bio_associate_blkcg.

Changing as suggested.

Thanks,
Paolo

> Cheers,
> Jeff
>
> >   	return split;
> >   }
> >   EXPORT_SYMBOL(bio_split);
> > @@ -2016,6 +2018,19 @@ 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
> > + */
> > +int bio_clone_blkcg_association(struct bio *dst, struct bio *src)
> > +{
> > +	if (src->bi_css)
> > +		return bio_associate_blkcg(dst, src->bi_css);
> > +
> > +	return 0;
> > +}
> > +
> >   #endif /* CONFIG_BLK_CGROUP */
> >
> >   static void __init biovec_init_slabs(void)
> > diff --git a/include/linux/bio.h b/include/linux/bio.h
> > index 6b7481f..8535f81 100644
> > --- a/include/linux/bio.h
> > +++ b/include/linux/bio.h
> > @@ -527,11 +527,13 @@ 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);
> > +int 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) { }
> > +int bio_clone_blkcg_association(struct bio *dst, struct bio *src) { return 0; }
> >   #endif	/* CONFIG_BLK_CGROUP */
> >
> >   #ifdef CONFIG_HIGHMEM