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

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Date:   Thu, 13 Aug 2020 15:45:58 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.org,
        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 Thu 13-08-20 15:27:15, Thomas Gleixner wrote:
> Michal Hocko <mhocko@...e.com> writes:
> > On Thu 13-08-20 11:58:40, Uladzislau Rezki wrote:
> > [...]
> >> Sorry for jumping in. We can rely on preemptable() for sure, if CONFIG_PREEMPT_RT
> >> is enabled, something like below:
> >> 
> >> if (IS_ENABLED_RT && preemptebale())
> >
> > Sure. I thought this was an RT specific thing that would be noop
> > otherwise.
> 
> Well, even if RT specific it would be still something returning either
> true or false unconditionally.
> 
> And guarding it with RT is not working either because then you are back
> to square one with the problem which triggered the discussion in the
> first place:
> 
> raw_spin_lock()
>   alloc()
>     if (RT && !preemptible())  <- False because RT == false
>     	goto bail;
> 
>     spin_lock(&zone->lock)  --> LOCKDEP complains
> 
> So either you convince Paul not to do that or you need to do something
> like I suggested in my other reply.

Can we somehow annotate the lock to be safe for nesting for lockdep?
-- 
Michal Hocko
SUSE Labs