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

[Joel Fernandes <joel@joelfernandes.org>]

On Sat, Feb 22, 2020 at 05:10:18PM -0800, Paul E. McKenney wrote:
[...] 
> > I was thinking a 2 fold approach (just thinking out loud..):
> > 
> > If kfree_call_rcu() is called in atomic context or in any rcu reader, then
> > use GFP_ATOMIC to grow an rcu_head wrapper on the atomic memory pool and
> > queue that.
> > 
> > Otherwise, grow an rcu_head on the stack of kfree_call_rcu() and call
> > synchronize_rcu() inline with it.
> > 
> > Use preemptible() andr task_struct's rcu_read_lock_nesting to differentiate
> > between the 2 cases.
> > 
> > Thoughts?
> 
> How much are we really losing by having an rcu_head in the structure,
> either separately or unioned over other fields?

It does seem a convenient API at first glance. Also seems like there are a
number of usecases now (ext4, vfree that Vlad mentioned, and all the other
users he mentioned, etc).

> > > > 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.
> > 
> > If *everything* runs out, you are pretty much going to OOM sooner or later
> > anyway :D. But I see what you mean. But the 'tradeoff' is RCU can free
> > head-less objects where possible.
> 
> Would you rather pay an rcu_head tax (in cases where it cannot share
> with other fields), or would you rather have states where you could free
> a lot of memory if only there was some memory to allocate to track the
> memory to be freed?

Depends on the usecase we could use the right API.

> But yes, as you suggested above, there could be an API similar to the
> userspace RCU library's API that usually just queues the callback but
> sometimes sleeps for a full grace period.  If enough people want this,
> it should not be hard to set up.

Sounds good!

thanks,

 - Joel