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

[Oscar Salvador <osalvador@suse.de>]

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 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