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

From: Michal Hocko <mhocko@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: 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
Date: Tue, 31 Mar 2020 17:34:50 +0200	[thread overview]
Message-ID: <20200331153450.GM30449@dhcp22.suse.cz> (raw)
In-Reply-To: <20200331145806.GB236678@google.com>

On Tue 31-03-20 10:58:06, Joel Fernandes wrote:
[...]
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 4be763355c9fb..965deefffdd58 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj)
> >  
> >  	if (!ptr)
> >  		ptr = kmalloc(sizeof(unsigned long *) +
> > -				sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN);
> > +				sizeof(struct rcu_head), GFP_MEMALLOC);
> 
> Just to add, the main requirements here are:
> 1. Allocation should be bounded in time.
> 2. Allocation should try hard (possibly tapping into reserves)
> 3. Sleeping is Ok but should not affect the time bound.

__GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to
memory reserves regarless of the sleeping status.

Using __GFP_MEMALLOC is quite dangerous because it can deplete _all_ the
memory. What does prevent the above code path to do that?
If a partial access to reserves is sufficient then why the existing
modifiers (mentioned above are not sufficient?
-- 
Michal Hocko
SUSE Labs