[PATCH v2 2/2] mm/hugetlb: allow overcommitting gigantic hugepages

[Usama Arif <usamaarif642@gmail.com>]

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.

The restriction was added in 2011 [1], which was before there was support
for reserving 1G hugepages at runtime.
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);
...

[1] https://git.zx2c4.com/linux-rng/commit/mm/hugetlb.c?id=adbe8726dc2a3805630d517270db17e3af86e526

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

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e74e41386b100..c0036c0775d75 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2243,7 +2243,7 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
 {
 	struct folio *folio = NULL;
 
-	if (hstate_is_gigantic(h))
+	if (hstate_is_gigantic_no_runtime(h))
 		return NULL;
 
 	spin_lock_irq(&hugetlb_lock);
@@ -4305,7 +4305,7 @@ 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_no_runtime(h))
 		return -EINVAL;
 
 	err = kstrtoul(buf, 10, &input);
@@ -5192,7 +5192,7 @@ 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_no_runtime(h))
 		return -EINVAL;
 
 	ret = proc_hugetlb_doulongvec_minmax(table, write, buffer, length, ppos,
-- 
2.47.3