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

[Uladzislau Rezki]

On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote:
> On Thu 01-10-20 21:26:26, Uladzislau Rezki wrote:
> > > 
> > > No, I meant going back to idea of new gfp flag, but adjust the implementation in
> > > the allocator (different from what you posted in previous version) so that it
> > > only looks at the flag after it tries to allocate from pcplist and finds out
> > > it's empty. So, no inventing of new page allocator entry points or checks such
> > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't
> > > affect existing fast paths.
> > >
> > OK. Now i see. Please have a look below at the patch, so we fully understand
> > each other. If that is something that is close to your view or not:
> > 
> > <snip>
> > t a/include/linux/gfp.h b/include/linux/gfp.h
> > index c603237e006c..7e613560a502 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -39,8 +39,9 @@ struct vm_area_struct;
> >  #define ___GFP_HARDWALL                0x100000u
> >  #define ___GFP_THISNODE                0x200000u
> >  #define ___GFP_ACCOUNT         0x400000u
> > +#define ___GFP_NO_LOCKS                0x800000u
> 
> Even if a new gfp flag gains a sufficient traction and support I am
> _strongly_ opposed against consuming another flag for that. Bit space is
> limited.
>
That is a valid point.

>
> Besides that we certainly do not want to allow craziness like
> __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we?
>
Obviously not. And it seems that the way of implementing of the
NO_LOCK logic would be easier(less code changes) and better if
it was defined like below(what you proposed later in this thread):

-#define __GFP_NO_LOCKS ((__force gfp_t)___GFP_NO_LOCKS)
+#define __GFP_NO_LOCKS ((__force gfp_t) 0)

That could imply that calling the page allocator with zero
argument would apply a further limitation - that is lock free.

--
Vlad Rezki