Re: [PATCH] mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios

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

> On May 22, 2025, at 13:34, Oscar Salvador <osalvador@suse.de> wrote:
> 
> On Thu, May 22, 2025 at 11:47:05AM +0800, Muchun Song wrote:
>> Thanks for fixing this problem. BTW, in order to catch future similar problem,
>> it is better to add WARN_ON into folio_hstate() to assert if hugetlb_lock
>> is not held when folio's reference count is zero. For this fix, LGTM.
> 
> Why cannot we put all the burden in alloc_and_dissolve_hugetlb_folio(),
> which will again check things under the lock?

I've also considered about this choice, because there is another similar
case in isolate_or_dissolve_huge_page() which could benefit from this
change. I am fine with both approaches. Anyway, adding an assertion into
folio_hstate() is an improvement for capturing invalid users in the future.
Because any user of folio_hstate() should hold a reference to folio or
hold the hugetlb_lock to make sure it returns a valid hstate for a hugetlb
folio.

Muchun,
Thanks.

> I mean, I would be ok to save cycles and check upfront in
> replace_free_hugepage_folios(), but the latter has only one user which
> is alloc_contig_range(), which is not really an expected-to-be optimized
> function.
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bd8971388236..b4d937732256 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2924,13 +2924,6 @@ int replace_free_hugepage_folios(unsigned long start_pfn, unsigned long end_pfn)
> 
>   	while (start_pfn < end_pfn) {
>   		folio = pfn_folio(start_pfn);
> - 		if (folio_test_hugetlb(folio)) {
> - 			h = folio_hstate(folio);
> - 		} else {
> - 			start_pfn++;
> - 			continue;
> - 		}
> -
>   		if (!folio_ref_count(folio)) {
>   			ret = alloc_and_dissolve_hugetlb_folio(h, folio,
>         						&isolate_list);
> 
> 
> 
> -- 
> Oscar Salvador
> SUSE Labs