Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Wed, 1 Apr 2020 15:08:16 +0200
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org,
        neilb@suse.com, vbabka@suse.cz, mgorman@suse.de,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free
 memory pattern
Message-ID: <20200401130816.GA1320@pc636>
References: <20200331131628.153118-1-joel@joelfernandes.org>
 <20200331145806.GB236678@google.com>
 <20200331153450.GM30449@dhcp22.suse.cz>
 <20200331161215.GA27676@pc636>
 <20200401070958.GB22681@dhcp22.suse.cz>
 <20200401123230.GB32593@pc636>
 <20200401125503.GJ22681@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200401125503.GJ22681@dhcp22.suse.cz>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Apr 01, 2020 at 02:55:03PM +0200, Michal Hocko wrote:
> On Wed 01-04-20 14:32:30, Uladzislau Rezki wrote:
> > On Wed, Apr 01, 2020 at 09:09:58AM +0200, Michal Hocko wrote:
> > > On Tue 31-03-20 18:12:15, Uladzislau Rezki wrote:
> > > > > 
> > > > > __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to
> > > > > memory reserves regarless of the sleeping status.
> > > > > 
> > > > Michal, just one question here regarding proposed flags. Can we also
> > > > tight it with __GFP_RETRY_MAYFAIL flag? Means it also can repeat a few
> > > > times in order to increase the chance of being success.
> > > 
> > > yes, __GFP_RETRY_MAYFAIL is perfectly valid with __GFP_ATOMIC. Please
> > > note that __GFP_ATOMIC, despite its name, doesn't imply an atomic
> > > allocation which cannot sleep. Quite confusing, I know. A much better
> > > name would be __GFP_RESERVES or something like that.
> > > 
> > OK. Then we can use GFP_ATOMIC | __GFP_RETRY_MAYFAIL to try in more harder
> > way.
> 
> Please note the difference between __GFP_ATOMIC and GFP_ATOMIC. The
> later is a highlevel flag to use for atomic contexts. The former is an
> explicit way to give an access to memory reserves. I am not familiar
> with your code but if you have an existing gfp context coming from the
> caller then just do (gfp | __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL).
> If you do not have any gfp then decide based on whether the current
> context is allowed to sleep
> 	gfp = GFP_KERNEL | __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL;
> 	if (!sleepable)
> 		gfp &= ~__GFP_DIRECT_RECLAIM;

We call it from atomic context, so we can not sleep, also we do not have
any existing context coming from the caller. I see that GFP_ATOMIC is high-level
flag and is differ from __GFP_ATOMIC. It is defined as:

#define GFP_ATOMIC (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)

so basically we would like to have __GFP_KSWAPD_RECLAIM that is included in it,
because it will also help in case of high memory pressure and wake-up kswapd to
reclaim memory.

We also can extract:

__GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL | __GFP_KSWAPD_RECLAIM

but that is longer then

GFP_ATMOC |  __GFP_RETRY_MAYFAIL

Am i missing something?

Thank you, Michal!

--
Vlad Rezki