mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap

Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
freeing unused vmemmap pages associated with each hugetlb page on boot.

We disable PMD mapping of vmemmap pages for x86-64 arch when this feature
is enabled.  Because vmemmap_remap_free() depends on vmemmap being base
page mapped.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Barry Song <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Tested-by: Chen Huang <[email protected]>
Tested-by: Bodeddula Balasubramaniam <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: HORIGUCHI NAOYA <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joao Martins <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Oliver Neukum <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Pawan Gupta <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Xiongchun Duan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 566c4b9af3cd..b787a9953f2e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1567,6 +1567,23 @@
 			Documentation/admin-guide/mm/hugetlbpage.rst.
 			Format: size[KMG]
 
+	hugetlb_free_vmemmap=
+			[KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+			enabled.
+			Allows heavy hugetlb users to free up some more
+			memory (6 * PAGE_SIZE for each 2MB hugetlb page).
+			This feauture is not free though. Large page
+			tables are not used to back vmemmap pages which
+			can lead to a performance degradation for some
+			workloads. Also there will be memory allocation
+			required when hugetlb pages are freed from the
+			pool which can lead to corner cases under heavy
+			memory pressure.
+			Format: { on | off (default) }
+
+			on:  enable the feature
+			off: disable the feature
+
 	hung_task_panic=
 			[KNL] Should the hung task detector generate panics.
 			Format: 0 | 1