linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Sat, 15 Aug 2020 01:40:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "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 1/2] mm: Add __GFP_NO_LOCKS flag

On Fri, Aug 14 2020 at 23:52, Peter Zijlstra wrote:
> On Fri, Aug 14, 2020 at 01:41:40PM -0700, Paul E. McKenney wrote:
>> > And that enforces the GFP_NOLOCK allocation mode or some other solution
>> > unless you make a new rule that calling call_rcu() is forbidden while
>> > holding zone lock or any other lock which might be nested inside the
>> > GFP_NOWAIT zone::lock held region.
>> 
>> Again, you are correct.  Maybe the forecasted weekend heat will cause
>> my brain to hallucinate a better solution, but in the meantime, the
>> GFP_NOLOCK approach looks good from this end.
>
> So I hate __GFP_NO_LOCKS for a whole number of reasons:
>
>  - it should be called __GFP_LOCKLESS if anything
>  - it sprinkles a bunch of ugly branches around the allocator fast path
>  - it only works for order==0
>
> Combined I really odn't think this should be a GFP flag. How about a
> special purpose allocation function, something like so..

No. No. No.

It's not requried at all after the context got set straight and my brain
started working again.

There is no need to provide such a thing which tries to "optimize"
unfixable problems and as a consequence makes other people use it for
the completely wrong reasons all over the place.

We have plenty of that stuff already. No need for more ...

Thanks,

        tglx