linux-kernel - Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

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Message-ID: <20200401125503.GJ22681@dhcp22.suse.cz>
Date:   Wed, 1 Apr 2020 14:55:03 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        rcu@...r.kernel.org, willy@...radead.org, peterz@...radead.org,
        neilb@...e.com, vbabka@...e.cz, mgorman@...e.de,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free
 memory pattern

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;
-- 
Michal Hocko
SUSE Labs