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

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Message-ID: <20200811102124.GH4793@dhcp22.suse.cz>
Date:   Tue, 11 Aug 2020 12:21:24 +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 Tue 11-08-20 11:18:07, Uladzislau Rezki wrote:
> On Mon, Aug 10, 2020 at 09:25:25PM +0200, Michal Hocko wrote:
> > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> > > > On Sun 09-08-20 22:43:53, Uladzislau Rezki (Sony) wrote:
[...]
> > > As i described before, calling the __get_free_page(0) with 0 as argument
> > > will solve the (a). How correctly is it? From my point of view the logic
> > > that bypass the wakeup path should be explicitly defined.
> > 
> > gfp_mask == 0 is GFP_NOWAIT (aka an atomic allocation request) which
> > doesn't wake up kswapd. So if the wakeup is a problem then this would be
> > a way to go.
> > 
> What do you mean Michal? gfp_mask 0 != GFP_NOWAIT:
> 
> #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM)
> 
> it does wakeup of the kswapd. Or am i missing something? Please comment.
> If we are about to avoid the kswapd, should we define something special?
> 
> #define GFP_NOWWAKE_KSWAPD 0

Sorry, I was more cryptic than necessary. What I meant is that
GFP_NOWAIT is the basic non-sleepable allocation. It does wake up
kswapd but a lack of it can be expressed GFP_NOWAIT & ~__GFP_KSWAPD_RECLAIM
which is 0, now. The mouthfull variant is better for future
maintainability.
-- 
Michal Hocko
SUSE Labs