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

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Date:   Tue, 11 Aug 2020 10:19:17 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     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>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        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 Mon 10-08-20 21:25:26, Michal Hocko wrote:
> On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
[...]
> > The problem that i see is we can not use the page allocator from atomic
> > contexts, what is our case:
> > 
> > <snip>
> >     local_irq_save(flags) or preempt_disable() or raw_spinlock();
> >     __get_free_page(GFP_ATOMIC);
> > <snip>
> > 
> > So if we can convert the page allocator to raw_* lock it will be appreciated,
> > at least from our side, IMHO, not from RT one. But as i stated above we need
> > to sort raised questions out if converting is done.
> > 
> > What is your view?
> 
> To me it would make more sense to support atomic allocations also for
> the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> work for atomic context in RT sounds subtle and wrong.

I was thinking about this some more. I still think the above would be a
reasonable goal we should try to achieve. If for not other then for
future maintainability (especially after the RT patchset is merged).
I have tried to search for any known problems/attempts to make
zone->lock raw but couldn't find anything. Maybe somebody more involved
in RT world have something to say about that.

Anyway, if the zone->lock is not a good fit for raw_spin_lock then the
only way I can see forward is to detect real (RT) atomic contexts and
bail out early before taking the lock in the allocator for NOWAIT/ATOMIC
requests.
-- 
Michal Hocko
SUSE Labs