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

[Uladzislau Rezki]

On Wed, Sep 30, 2020 at 06:46:00PM +0200, Michal Hocko wrote:
> On Wed 30-09-20 15:39:54, Uladzislau Rezki wrote:
> > On Wed, Sep 30, 2020 at 02:44:13PM +0200, Michal Hocko wrote:
> > > On Wed 30-09-20 14:35:35, Uladzislau Rezki wrote:
> > > > On Wed, Sep 30, 2020 at 11:27:32AM +0200, Michal Hocko wrote:
> > > > > On Tue 29-09-20 18:25:14, Uladzislau Rezki wrote:
> > > > > > > > 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).
> > > > > > >   *
> > > > > > >   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
> > > > > > >   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> > > > > > > 
> > > > > > To me it is clear. But also above conflicting statement:
> > > > > > 
> > > > > > <snip>
> > > > > > %GFP_ATOMIC users can not sleep and need the allocation to succeed. A %lower
> > > > > > <snip>
> > > > > > 
> > > > > > should be rephrased, IMHO.
> > > > > 
> > > > > Any suggestions? Or more specifics about which part is conflicting? It
> > > > > tries to say that there is a higher demand to succeed even though the
> > > > > context cannot sleep to take active measures to achieve that. So the
> > > > > only way to achieve that is to break the watermakrs to a certain degree
> > > > > which is making them more "higher class" than other allocations.
> > > > > 
> > > > Michal, i had only one concern about it. It says that %GFP_ATOMIC users
> > > > can not sleep, i.e. callers know that they are in atomic, thus no any
> > > > sleeping, but the chose they make will force them to sleep.
> > > 
> > > I am not sure I follow you here. Do you mean they will be forced to
> > > sleep with PREEMPT_RT?
> > > 
> > Exactly :)
> 
> We can make that more specific once RT patchset is merged. As of now
> this is not the thing in the Linus tree. I believe there will be more to
> clarify about atomic contexts in the RT tree as it means something else
> than people are used to think.
> 
Fair enough. Currently we have only the Kconfig CONFIG_PREEMPT_RT
symbol, so RT patches are still not in place.

--
Vlad Rezki