Re: [PATCH V2] mm/hugetlb: Make __NR_USED_SUBPAGE check conditional

[Muchun Song <muchun.song@linux.dev>]

> On Dec 3, 2024, at 10:32, Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> The HugeTLB order check against __NR_USED_SUBPAGE is required only when
> HUGETLB_PAGE_OPTIMIZE_VMEMMAP is enabled. Hence BUG_ON() trigger should
> happen only when applicable.
> 
> Cc: Muchun Song <muchun.song@linux.dev>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This patch applies on v6.13-rc1
> 
> Changes in V2:
> 
> - Fixed #ifdef with CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP per Oscar
> 
> Changes in V1:
> 
> https://lore.kernel.org/all/20241202090728.78935-1-anshuman.khandual@arm.com/
> 
> mm/hugetlb.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ea2ed8e301ef..e6a5b21e3578 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4513,11 +4513,13 @@ void __init hugetlb_add_hstate(unsigned int order)
> 	struct hstate *h;
> 	unsigned long i;
> 
> - 	if (size_to_hstate(PAGE_SIZE << order)) {
> + 	if (size_to_hstate(PAGE_SIZE << order))
> 		return;
> - 	}
> +
> 	BUG_ON(hugetlb_max_hstate >= HUGE_MAX_HSTATE);
> +#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> 	BUG_ON(order < order_base_2(__NR_USED_SUBPAGE));

Hi Anshuman,

__NR_USED_SUBPAGE indicates how many struct pages are used to store
extra metadata for a HugeTLB page. So we need to make sure the order
of a HugeTLB page should be bigger than or equal to order_base_2(__NR_USED_SUBPAGE).
So It is not related to HVO. I don't think the changes make sense.

Thanks.

> +#endif
> 	h = &hstates[hugetlb_max_hstate++];
> 	__mutex_init(&h->resize_lock, "resize mutex", &h->resize_key);
> 	h->order = order;
> -- 
> 2.30.2
>