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

[Hugh Dickins]

On Wed, 9 Feb 2022, Geert Uytterhoeven wrote:
> On Wed, Feb 9, 2022 at 4:38 PM Hugh Dickins <hughd@xxxxxxxxxx> 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