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

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Message-ID: <20200813111505.GG9477@dhcp22.suse.cz>
Date:   Thu, 13 Aug 2020 13:15:05 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, 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 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.

> Also i have a question about pcp-lists. Can we introduce and use all allowed 
> MIGRATE_PCPTYPES? If called with GFP_RT_SAFE? If not please elaborate.

Yes we can. This depends on the provided gfp_mask. Many gfp flags will
be meaningless for such a context but essentially all we care about is
that ((gfp_mask & __GFP_RECLAIM) == GFP_RT_SAFE) for the checking
purpose. We can sort out all these details if this is decided to be the
right way to do. My (pseudo) patch was mostly to show the direction I've
had in mind for easier further discussion.

-- 
Michal Hocko
SUSE Labs