[PATCH] mm/thp: add CONFIG_TRANSPARENT_HUGEPAGE_NEVER option

[Dmytro Maluka <dmaluka@chromium.org>]

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.

[1] https://lore.kernel.org/all/202211301651462590168@zte.com.cn/

Signed-off-by: Dmytro Maluka <dmaluka@chromium.org>
---
 mm/Kconfig | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 89971a894b60..ec2d5841c9dc 100644
--- 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
+	  enabled at runtime via sysfs.
 endchoice
 
 config THP_SWAP
-- 
2.43.0.rc2.451.g8631bc7472-goog