linux-kernel - Re: [PATCH for-mm 1/2] mm/internal: Implement no-op mlock_page_drain() for !CONFIG

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Date:   Wed, 9 Feb 2022 20:37:16 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     Hugh Dickins <hughd@...gle.com>, SeongJae Park <sj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH for-mm 1/2] mm/internal: Implement no-op mlock_page_drain()
 for !CONFIG_MMU

On Wed, 9 Feb 2022, Geert Uytterhoeven wrote:
> On Wed, Feb 9, 2022 at 4:38 PM Hugh Dickins <hughd@...gle.com> wrote:
> > The thing is, SeongJae's patch makes me wonder, why did it not need a
> > !CONFIG_MMU definition for need_mlock_page_drain() too?  That's because
> > mm/swap.c's call to it is under an #ifdef CONFIG_SMP, and I imagine that
> > CONFIG_MMU=n usually goes along with (but does not necessarily imply?)
> > CONFIG_SMP=n.  It'll be safer to add a need_mlock_page_drain() stub too.
> 
> RISC-V K210 is SMP without MMU.

Thanks Geert, that makes it very clear that we also want the stub for
need_mlock_page_drain().  I'll follow now with update of SeongJae's patch.

My fear of wider implications of not having CONFIG_SMP turned out to
be a false alarm.  The UP lru_add_drain_cpu() calls mlock_page_drain()
just like the SMP one does: the difference between them is merely that
the UP case doesn't need an efficient way for asking in advance whether
drain on another cpu will be needed.

Hugh