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

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Date:   Wed, 1 Apr 2020 15:22:58 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        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

> > 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
> 
> OK, if you are always in the atomic context then GFP_ATOMIC is
> sufficient. __GFP_RETRY_MAYFAIL will make no difference for allocations
> which do not reclaim (and thus not retry). Sorry this was not clear to
> me from the previous description.
> 
Ahh. OK. Then adding __GFP_RETRY_MAYFAIL to GFP_ATOMIC will not make any effect.

Thank you for your explanation!

--
Vlad Rezki