linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Message-ID: <20200815084250.GN3982@worktop.programming.kicks-ass.net>
Date:   Sat, 15 Aug 2020 10:42:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     paulmck@...nel.org, Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Sat, Aug 15, 2020 at 01:14:53AM +0200, Thomas Gleixner wrote:

> #1 trivial fix is to force switching to an high prio thread or a soft
>    interrupt which does the allocation

Yeah, push the alocation out to another context. I did consider it, but
why bother?

Also, raising a softirq can't be done from every context, that's a whole
new problem. You can do irq_work I suppose, but not all architectures
support the self-IPI yet.

All in all, it's just more complexity than the fairly trivial
__alloc_page_lockless().

Whichever way around, we can't rely on the allocation.