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

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Message-ID: <20201002071123.GB20872@dhcp22.suse.cz>
Date:   Fri, 2 Oct 2020 09:11:23 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     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>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

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.  Besides that we certainly do not want to allow craziness like
__GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we?
-- 
Michal Hocko
SUSE Labs