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 17:46:18 +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

> > > 
> > > 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!
> 
> Welcome. I wish all those gfp flags were really clear but I fully
> understand that people who are not working with MM regurarly might find
> it confusing. Btw. have __GFP_RETRY_MAYFAIL is documented in gfp.h and
> it is documented as the reclaim modifier which should imply that it has
> no effect when the reclaim is not allowed which is the case for any non
> sleeping allocation. If that relation was not immediately obvious then I
> think we need to make it explicit. Would you find it useful?
> 
> E.g.
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index e3ab1c0d9140..8f09cefdfa7b 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -127,6 +127,8 @@ struct vm_area_struct;
>   *
>   * Reclaim modifiers
>   * ~~~~~~~~~~~~~~~~~
> + * Please note that all the folloging flags are only applicable to sleepable
> + * allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
>   *
>   * %__GFP_IO can start physical IO.
>   *
That would be definitely clear for me!

--
Vlad Rezki