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Date:   Mon, 4 Dec 2023 12:15:24 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Dmytro Maluka <dmaluka@chromium.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/thp: add CONFIG_TRANSPARENT_HUGEPAGE_NEVER option
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On Mon, 4 Dec 2023 20:57:33 +0100 Dmytro Maluka <dmaluka@chromium.org> wrote:

> On Mon, Dec 04, 2023 at 11:13:01AM -0800, Andrew Morton wrote:
> > On Mon,  4 Dec 2023 17:32:54 +0100 Dmytro Maluka <dmaluka@chromium.org> wrote:
> > 
> > > Add an option to disable transparent hugepages by default, in line with
> > > the existing transparent_hugepage=never command line setting.
> > > 
> > > Rationale: khugepaged has its own non-negligible memory cost even if it
> > > is not used by any applications, since it bumps up vm.min_free_kbytes to
> > > its own required minimum in set_recommended_min_free_kbytes(). For
> > > example, on a machine with 4GB RAM, with 3 mm zones and pageblock_order
> > > == MAX_ORDER, starting khugepaged causes vm.min_free_kbytes increase
> > > from 8MB to 132MB.
> > > 
> > > So if we use THP on machines with e.g. >=8GB of memory for better
> > > performance, but avoid using it on lower-memory machines to avoid its
> > > memory overhead, then for the same reason we also want to avoid even
> > > starting khugepaged on those <8GB machines. So with
> > > CONFIG_TRANSPARENT_HUGEPAGE_NEVER we can use the same kernel image on
> > > both >=8GB and <8GB machines, with THP support enabled but khugepaged
> > > not started by default. The userspace can then decide to enable THP
> > > (i.e. start khugepaged) via sysfs if needed, based on the total amount
> > > of memory.
> > > 
> > > This could also be achieved with the existing transparent_hugepage=never
> > > setting in the kernel command line instead. But it seems cleaner to
> > > avoid tweaking the command line for such a basic setting.
> > > 
> > > P.S. I see that CONFIG_TRANSPARENT_HUGEPAGE_NEVER was already proposed
> > > in the past [1] but without an explanation of the purpose.
> > > 
> > > ...
> > >
> > > --- a/mm/Kconfig
> > > +++ b/mm/Kconfig
> > > @@ -859,6 +859,12 @@ choice
> > >  	  madvise(MADV_HUGEPAGE) but it won't risk to increase the
> > >  	  memory footprint of applications without a guaranteed
> > >  	  benefit.
> > > +
> > > +	config TRANSPARENT_HUGEPAGE_NEVER
> > > +		bool "never"
> > > +	help
> > > +	  Disabling Transparent Hugepage by default. It can still be
> > 
> > s/Disabling/Disable/
> 
> It is in line with the descriptions of TRANSPARENT_HUGEPAGE_ALWAYS and
> TRANSPARENT_HUGEPAGE_MADVISE: "Enabling Transparent Hugepage ..."

Those are incorrect also.

> > > +	  enabled at runtime via sysfs.
> > >  endchoice
> > 
> > The patch adds the config option but doesn't use it?
> 
> I should have been more precise: it is not a new option but a new choice
> for CONFIG_TRANSPARENT_HUGEPAGE, in addition to the existing ALWAYS and
> MADVISE choices. In mm/huge_memory.c in the declaration of the
> transparent_hugepage_flags variable, if either ALWAYS or MADVISE is
> chosen, transparent_hugepage_flags is initialized with such a value
> that makes khugepaged being started by default during bootup. This patch
> allows enabling CONFIG_TRANSPARENT_HUGEPAGE without setting either
> ALWAYS or MADVISE, so that transparent_hugepage_flags is initialized
> with such a value that khugepaged is not started by default.

OK, thanks.