Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css

[Jan Kara]

Hi!

On Thu 20-05-21 11:45:23, Muchun Song wrote:
> It seems like this patch has not been added to the linux-next
> tree. Can anyone help with this? Thanks.

Muchun, did someone pickup this patch? I don't see it merged so unless
somebody yells, I'll pick it up to my tree and send it to Linus for rc2.

								Honza

> 
> On Fri, Apr 2, 2021 at 5:13 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > The caller of wb_get_create() should pin the memcg, because
> > wb_get_create() relies on this guarantee. The rcu read lock
> > only can guarantee that the memcg css returned by css_from_id()
> > cannot be released, but the reference of the memcg can be zero.
> >
> >   rcu_read_lock()
> >   memcg_css = css_from_id()
> >   wb_get_create(memcg_css)
> >       cgwb_create(memcg_css)
> >           // css_get can change the ref counter from 0 back to 1
> >           css_get(memcg_css)
> >   rcu_read_unlock()
> >
> > Fix it by holding a reference to the css before calling
> > wb_get_create(). This is not a problem I encountered in the
> > real world. Just the result of a code review.
> >
> > Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> > Changelog in v3:
> >  1. Do not change GFP_ATOMIC.
> >  2. Update commit log.
> >
> >  Thanks for Michal's review and suggestions.
> >
> > Changelog in v2:
> >  1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew.
> >
> >
> >  fs/fs-writeback.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 3ac002561327..dedde99da40d 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -506,9 +506,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
> >         /* find and pin the new wb */
> >         rcu_read_lock();
> >         memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> > -       if (memcg_css)
> > -               isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> > +       if (memcg_css && !css_tryget(memcg_css))
> > +               memcg_css = NULL;
> >         rcu_read_unlock();
> > +       if (!memcg_css)
> > +               goto out_free;
> > +
> > +       isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> > +       css_put(memcg_css);
> >         if (!isw->new_wb)
> >                 goto out_free;
> >
> > --
> > 2.11.0
> >
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR