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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Mon, 5 Oct 2020 15:58:54 +0200
To:     Michal Hocko <mhocko@suse.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Joel Fernandes <joel@joelfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Message-ID: <20201005135854.GA17959@pc636>
References: <20200918194817.48921-1-urezki@gmail.com>
 <20200918194817.48921-3-urezki@gmail.com>
 <38f42ca1-ffcd-04a6-bf11-618deffa897a@suse.cz>
 <20200929220742.GB8768@pc636>
 <795d6aea-1846-6e08-ac1b-dbff82dd7133@suse.cz>
 <20201001192626.GA29606@pc636>
 <20201002071123.GB20872@dhcp22.suse.cz>
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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