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

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Date:   Fri, 2 Oct 2020 11:58:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Michal Hocko <mhocko@...e.com>,
        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>,
        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 10:45:02AM +0100, Mel Gorman wrote:
> On Fri, Oct 02, 2020 at 11:07:29AM +0200, Peter Zijlstra wrote:
> > On Fri, Oct 02, 2020 at 09:50:14AM +0100, Mel Gorman wrote:
> > > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote:
> > 
> > > > > +#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.
> > 
> > This is one of the reasons I did a separate allocation function. No GFP
> > flag to leak into general usage.
> > 
> 
> Even a specific function with a hint that "this is for RCU only" will
> not prevent abuse.

Not exporting it for modules helps, but yes.

> > > > 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.
> > 
> > Isn't that what we have CONFIG_DEBUG_VM for?
> 
> It's enabled by default by enough distros that adding too many checks
> is potentially painful. Granted it would be missed by most benchmarking
> which tend to control allocations from userspace but a lot of performance
> problems I see are the "death by a thousand cuts" variety.

Oh quite agreed, aka death by accounting. But if people are enabling
DEBUG options in production kernels, there's something wrong, no?

Should we now go add CONFIG_REALLY_DEBUG_STAY_AWAY_ALREADY options?