Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations

Paul E. McKenney
2020-02-21T00:30:37Z

On Wed, Feb 19, 2020 at 11:52:33PM -0500, Theodore Y. Ts'o wrote:
> On Tue, Feb 18, 2020 at 06:08:57PM +0100, Uladzislau Rezki wrote:
> > now it becomes possible to use it like:
> > ...
> > void *p = kvmalloc(PAGE_SIZE);
> > kvfree_rcu(p);
> > ...
> > also have a look at the example in the mm/list_lru.c diff.
>
> I certainly like the interface, thanks! I'm going to be pushing
> patches to fix this using ext4_kvfree_array_rcu() since there are a
> number of bugs in ext4's online resizing which appear to be hitting
> multiple cloud providers (with reports from both AWS and GCP) and I
> want something which can be easily backported to stable kernels.
>
> But once kvfree_rcu() hits mainline, I'll switch ext4 to use it, since
> your kvfree_rcu() is definitely more efficient than my expedient
> jury-rig.
>
> I don't feel entirely competent to review the implementation, but I do
> have one question. It looks like the rcutiny implementation of
> kfree_call_rcu() isn't going to do the right thing with kvfree_rcu(p).
> Am I missing something?

Good catch! I believe that rcu_reclaim_tiny() would need to do
kvfree() instead of its current kfree().

Vlad, anything I am missing here?

Thanx, Paul

> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 045c28b71f4f..a12ecc1645fa 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -34,7 +34,7 @@ static inline void synchronize_rcu_expedited(void)
> > synchronize_rcu();
> > }
> >
> > -static inline void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > +static inline void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func, void *ptr)
> > {
> > call_rcu(head, func);
> > }
>
> Thanks,
>
> - Ted