[PATCH] mm: hugetlb: Add Kconfig option to set default nr_overcommit_hugepages

[Josh Triplett <josh@joshtriplett.org>]

The default kernel configuration does not allow any huge page allocation
until after setting nr_hugepages or nr_overcommit_hugepages to a
non-zero value; without setting those, mmap attempts with MAP_HUGETLB
will always fail with -ENOMEM. nr_overcommit_hugepages allows userspace
to attempt to allocate huge pages at runtime, succeeding if the kernel
can find or assemble a free huge page.

Provide a Kconfig option to make nr_overcommit_hugepages default to
unlimited, which permits userspace to always attempt huge page
allocation on a best-effort basis. This makes it easier and more
worthwhile for random applications and libraries to opportunistically
attempt MAP_HUGETLB allocations without special configuration.

In particular, current versions of liburing with IORING_SETUP_NO_MMAP
attempt to allocate the rings in a huge page. This seems likely to lead
to more applications and libraries attempting to use huge pages.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 mm/Kconfig   | 14 ++++++++++++++
 mm/hugetlb.c |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 7672a22647b4..32c13610c5c4 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -824,6 +824,20 @@ config READ_ONLY_THP_FOR_FS
 
 endif # TRANSPARENT_HUGEPAGE
 
+config HUGEPAGE_OVERCOMMIT_DEFAULT_UNLIMITED
+	bool "Allow huge page allocation attempts by default"
+	depends on HUGETLB_PAGE
+	help
+	  By default, the kernel does not allow any huge page allocation until
+	  after setting nr_hugepages or nr_overcommit_hugepages to a non-zero
+	  value. nr_overcommit_hugepages allows userspace to attempt to
+	  allocate huge pages at runtime, succeeding if the kernel can find or
+	  assemble a free huge page.
+
+	  Enable this option to make nr_overcommit_hugepages default to
+	  unlimited, which permits userspace to always attempt hugepage
+	  allocation.
+
 #
 # UP and nommu archs use km based percpu allocator
 #
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f154019e6b84..65abbe254e10 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4305,6 +4305,8 @@ void __init hugetlb_add_hstate(unsigned int order)
 	mutex_init(&h->resize_lock);
 	h->order = order;
 	h->mask = ~(huge_page_size(h) - 1);
+	if (IS_ENABLED(CONFIG_HUGEPAGE_OVERCOMMIT_DEFAULT_UNLIMITED))
+		h->nr_overcommit_huge_pages = ULONG_MAX;
 	for (i = 0; i < MAX_NUMNODES; ++i)
 		INIT_LIST_HEAD(&h->hugepage_freelists[i]);
 	INIT_LIST_HEAD(&h->hugepage_activelist);
-- 
2.40.1