linux-kernel - Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page

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Message-ID: <20201002085014.GC3227@techsingularity.net>
Date:   Fri, 2 Oct 2020 09:50:14 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

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 definitely true. I'm not happy with the GFP flag at all, the
comment is at best a damage limiting move. It still would be better for
a memory pool to be reserved and sized for critical allocations.

> Besides that we certainly do not want to allow craziness like
> __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we?

That would deserve to be taken to a dumpster and set on fire. The flag
combination could be checked in the allocator but the allocator path fast
paths are bad enough already.

-- 
Mel Gorman
SUSE Labs