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

[Ge Yang <yangge1116@126.com>]

在 2025/5/22 13:34, Oscar Salvador 写道:
> 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);
> 
>   
> 
It seems that we cannot simply remove the folio_test_hugetlb() check. 
The reasons are as follows:

1）If we remove it, we will be unable to obtain the hstat corresponding 
to the folio, and consequently, we won't be able to call 
alloc_and_dissolve_hugetlb_folio().

2）The alloc_and_dissolve_hugetlb_folio() function is also called within 
the isolate_or_dissolve_huge_folio() function. However, the 
folio_test_hugetlb() check within the isolate_or_dissolve_huge_folio() 
function cannot be removed.