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

[Shakeel Butt]

On Thu, Oct 09, 2025 at 06:24:31PM +0100, Usama Arif wrote:
> 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@xxxxxxxxx>

Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>