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

[Paul E. McKenney]

On Fri, Feb 21, 2020 at 02:14:55PM +0100, Uladzislau Rezki wrote:
> On Thu, Feb 20, 2020 at 04:30:35PM -0800, Paul E. McKenney wrote:
> > 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?
> >
> Yes something like that. There are some open questions about
> realization, when it comes to tiny RCU. Since we are talking
> about "headless" kvfree_rcu() interface, i mean we can not link
> freed "objects" between each other, instead we should place a
> pointer directly into array that will be drained later on.
> 
> It would be much more easier to achieve that if we were talking
> about the interface like: kvfree_rcu(p, rcu), but that is not our
> case :)
> 
> So, for CONFIG_TINY_RCU we should implement very similar what we
> have done for CONFIG_TREE_RCU or just simply do like Ted has done
> with his
> 
> void ext4_kvfree_array_rcu(void *to_free)
> 
> i mean:
> 
>    local_irq_save(flags);
>    struct foo *ptr = kzalloc(sizeof(*ptr), GFP_ATOMIC);
> 
>    if (ptr) {
>            ptr->ptr = to_free;
>            call_rcu(&ptr->rcu, kvfree_callback);
>    }
>    local_irq_restore(flags);

We really do still need the emergency case, in this case for when
kzalloc() returns NULL.  Which does indeed mean an rcu_head in the thing
being freed.  Otherwise, you end up with an out-of-memory deadlock where
you could free memory only if you had memor to allocate.

> Also there is one more open question what to do if GFP_ATOMIC
> gets failed in case of having low memory condition. Probably
> we can make use of "mempool interface" that allows to have
> min_nr guaranteed pre-allocated pages. 

But we really do still need to handle the case where everything runs out,
even the pre-allocated pages.

							Thanx, Paul