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Date:   Tue, 5 Dec 2023 18:05:53 +0100
From:   Dmytro Maluka <dmaluka@chromium.org>
To:     Andrew Morton <akpm@linux-foundation.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, Dec 04, 2023 at 12:15:24PM -0800, Andrew Morton wrote:
> 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.

Ok, corrected in v2. Also clarified the changelog wrt your 2nd question.

> > > > +	  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.