mm/hugetlb: allow overcommitting gigantic hugepages

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);
...

Link: https://lkml.kernel.org/r/20251009172433.4158118-2-usamaarif642@gmail.com
Link: https://git.zx2c4.com/linux-rng/commit/mm/hugetlb.c?id=adbe8726dc2a3805630d517270db17e3af86e526
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Rik van Riel <riel@surriel.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bd53d367b18b7277d59668e4b5d3f22a4c7ab7bb..7774c286b3b751106983a76b8b5e48d4e68c4df7 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2223,7 +2223,7 @@ 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);
@@ -4285,7 +4285,7 @@ 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);
@@ -5172,7 +5172,7 @@ 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,