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

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Message-ID: <20200811111023.GA5544@pc636>
Date:   Tue, 11 Aug 2020 13:10:23 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     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>,
        "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, Aug 11, 2020 at 12:21:24PM +0200, Michal Hocko wrote:
> 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.
>
OK. I got it anyway. Just decided to clarify.

--
Vlad Rezki