Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

[Michal Hocko]

On Wed 30-09-20 13:03:29, Joel Fernandes wrote:
> On Wed, Sep 30, 2020 at 12:48 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Wed 30-09-20 11:25:17, Joel Fernandes wrote:
> > > On Fri, Sep 25, 2020 at 05:47:41PM +0200, Michal Hocko wrote:
> > > > On Fri 25-09-20 17:31:29, Uladzislau Rezki wrote:
> > > > > > > > >
> > > > > > > > > All good points!
> > > > > > > > >
> > > > > > > > > On the other hand, duplicating a portion of the allocator functionality
> > > > > > > > > within RCU increases the amount of reserved memory, and needlessly most
> > > > > > > > > of the time.
> > > > > > > > >
> > > > > > > >
> > > > > > > > But it's very similar to what mempools are for.
> > > > > > > >
> > > > > > > As for dynamic caching or mempools. It requires extra logic on top of RCU
> > > > > > > to move things forward and it might be not efficient way. As a side
> > > > > > > effect, maintaining of the bulk arrays in the separate worker thread
> > > > > > > will introduce other drawbacks:
> > > > > >
> > > > > > This is true but it is also true that it is RCU to require this special
> > > > > > logic and we can expect that we might need to fine tune this logic
> > > > > > depending on the RCU usage. We definitely do not want to tune the
> > > > > > generic page allocator for a very specific usecase, do we?
> > > > > >
> > > > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability
> > > > > to provide a memory service for contexts which are not allowed to
> > > > > sleep, RCU is part of them. Both flags used to provide such ability
> > > > > before but not anymore.
> > > > >
> > > > > Do you agree with it?
> > > >
> > > > Yes this sucks. But this is something that we likely really want to live
> > > > with. We have to explicitly _document_ that really atomic contexts in RT
> > > > cannot use the allocator. From the past discussions we've had this is
> > > > likely the most reasonable way forward because we do not really want to
> > > > encourage anybody to do something like that and there should be ways
> > > > around that. The same is btw. true also for !RT. The allocator is not
> > > > NMI safe and while we should be able to make it compatible I am not
> > > > convinced we really want to.
> > > >
> > > > Would something like this be helpful wrt documentation?
> > > >
> > > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > > > index 67a0774e080b..9fcd47606493 100644
> > > > --- a/include/linux/gfp.h
> > > > +++ b/include/linux/gfp.h
> > > > @@ -238,7 +238,9 @@ struct vm_area_struct;
> > > >   * %__GFP_FOO flags as necessary.
> > > >   *
> > > >   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
> > > > - * watermark is applied to allow access to "atomic reserves"
> > > > + * watermark is applied to allow access to "atomic reserves".
> > > > + * The current implementation doesn't support NMI and other non-preemptive context
> > > > + * (e.g. raw_spin_lock).
> > >
> > > I think documenting is useful.
> > >
> > > Could it be more explicit in what the issue is? Something like:
> > >
> > > * Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT
> > > systems. Therefore, the current low-level allocator implementation does not
> > > support being called from special contexts that are atomic on RT - such as
> > > NMI and raw_spin_lock. Due to these constraints and considering calling code
> > > usually has no control over the PREEMPT_RT configuration, callers of the
> > > allocator should avoid calling the allocator from these cotnexts even in
> > > non-RT systems.
> >
> > I do not mind documenting RT specific behavior but as mentioned in other
> > reply, this should likely go via RT tree for now. There is likely more
> > to clarify about atomicity for PREEMPT_RT.
> 
> I am sorry, I did not understand what you meant by something missing
> in Linus Tree. CONFIG_PREEMPT_RT is there.

OK, I was not aware we already CONFIG_PREEMPT_RT in the three. There is
still a lot from the RT patchset including sleeping spin locks that make
a real difference. Or maybe I haven't checked properly.

> Could you clarify? Also the CONFIG_PREEMPT_RT is the only thing
> driving this requirement for GFP_ATOMIC right? Or are there non-RT
> reasons why GFP_ATOMIC allocation cannot be done from
> NMI/raw_spin_lock ?

I have already sent a clarification patch [1]. NMI is not supported
regardless of the preemption mode. Hope this clarifies it a bit.

[1] http://lkml.kernel.org/r/20200929123010.5137-1-mhocko@xxxxxxxxxx

-- 
Michal Hocko
SUSE Labs