[PATCH] mm/hugetlb: allow overcommitting gigantic hugepages

[Usama Arif]

Currently, gigantic hugepages cannot use the overcommit mechanism
(nr_overcommit_hugepages), forcing users to permanently reserve memory via
nr_hugepages even when pages might not be actively used.

Remove this blanket restriction on gigantic hugepage overcommit.
This will bring the same benefits to gigantic pages as hugepages:

- Memory is only taken out of regular use when actually needed
- Unused surplus pages can be returned to the system
- Better memory utilization, especially with CMA backing which can
  significantly increase the changes of hugepage allocation

Without this patch:
echo 3 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages
bash: echo: write error: Invalid argument

With this patch:
echo 3 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages
./mmap_hugetlb_test
Successfully allocated huge pages at address: 0x7f9d40000000

cat mmap_hugetlb_test.c
...
    unsigned long ALLOC_SIZE = 3 * (unsigned long) HUGE_PAGE_SIZE;
    addr = mmap(NULL,
                ALLOC_SIZE, // 3GB
                PROT_READ | PROT_WRITE,
                MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB | MAP_HUGE_1GB,
                -1,
                0);

    if (addr == MAP_FAILED) {
        fprintf(stderr, "mmap failed: %s\n", strerror(errno));
        return 1;
    }
    printf("Successfully allocated huge pages at address: %p\n", addr);
...

Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
---
 mm/hugetlb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c07b7192aff26..93d0f8ae1fe84 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2232,7 +2232,7 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
 {
 	struct folio *folio = NULL;
 
-	if (hstate_is_gigantic(h))
+	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return NULL;
 
 	spin_lock_irq(&hugetlb_lock);
@@ -4294,7 +4294,7 @@ static ssize_t nr_overcommit_hugepages_store(struct kobject *kobj,
 	unsigned long input;
 	struct hstate *h = kobj_to_hstate(kobj, NULL);
 
-	if (hstate_is_gigantic(h))
+	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return -EINVAL;
 
 	err = kstrtoul(buf, 10, &input);
@@ -5181,7 +5181,7 @@ static int hugetlb_overcommit_handler(const struct ctl_table *table, int write,
 
 	tmp = h->nr_overcommit_huge_pages;
 
-	if (write && hstate_is_gigantic(h))
+	if (write && hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return -EINVAL;
 
 	ret = proc_hugetlb_doulongvec_minmax(table, write, buffer, length, ppos,
-- 
2.47.3