Re: [PATCH] mm, hugetlb: Reset mapping to TAIL_MAPPING before restoring vmemmap

[David Hildenbrand <david@redhat.com>]

On 15.04.25 09:23, David Hildenbrand wrote:
> On 15.04.25 07:47, Oscar Salvador wrote:
>> commit 4eeec8c89a0c ("mm: move hugetlb specific things in folio to page[3]")
>> shifted hugetlb specific stuff, and now mapping overlaps _hugetlb_cgroup field.
>>
>> _hugetlb_cgroup is set to NULL when preparing the hugetlb page in
>> init_new_hugetlb_folio().
>> For a better picture, this is page->mapping before and after the comming
>> for the first three tail pages:
>>
>> before:
>> page: fffff51a44358040  0000000000000000
>> page: fffff51a44358080  0000000000000000
>> page: fffff51a443580c0  dead000000000400
>>
>> after:
>> page: fffff1f0042b0040  0000000000000000
>> page: fffff1f0042b0080  fffff1f0042b0090
>> page: fffff1f0042b00c0  0000000000000000
>>
>> Tail#2 has fffff1f0042b0090 because of the _deferred_list initialization,
>> which was also shifted, but that is not a problem.
>>
>> For HVO, upon restoring that gets copied in some tail pages (reset_struct_pages)
>> and so those tail pages will not have TAIL_MAPPING set and the check
>> in free_tail_page_prepare() will fail:
>>
>>    kernel: BUG: Bad page state in process kworker/0:3  pfn:10ac40
>>    kernel: page does not match folio
>>    kernel: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x10ac40
>>    kernel: flags: 0x17ffffc0000000(node=0|zone=2|lastcpupid=0x1fffff)
>>    kernel: raw: 0017ffffc0000000 fffff1f0042b0000 0000000000000000 0000000000000000
>>    kernel: raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
>>    kernel: page dumped because: corrupted mapping in tail page
>>
>> Reset _hugetlb_cgroup to TAIL_MAPPING before restoring so tail pages have the
>> right value.
> 
> Hi,
> 
> To handle that for ordinary hugtlb alloc/free I added in that patch in free_tail_page_prepare():
> 
> 	case 3:
> 		/* the third tail page: hugetlb specifics overlap ->mappings */
> 		if (IS_ENABLED(CONFIG_HUGETLB_PAGE))
> 			break;
> 		fallthrough;
> 	default:
> 		if (page->mapping != TAIL_MAPPING) {
> 			bad_page(page, "corrupted mapping in tail page");
> 			goto out;
> 		}
> 		break;
> 	}
> 
> Now I am confused why that check doesn't catch that?
> 
> Apparently only a problem with HVO? Because I recall testing the ordinary alloc/free.

Ah, reading about the HVO hackery in the comment above NR_RESET_STRUCT_PAGE, might the following fix it?


diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 9a99dfa3c4958..27245e86df250 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -238,11 +238,11 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
   * struct page, the special metadata (e.g. page->flags or page->mapping)
   * cannot copy to the tail struct page structs. The invalid value will be
   * checked in the free_tail_page_prepare(). In order to avoid the message
- * of "corrupted mapping in tail page". We need to reset at least 3 (one
- * head struct page struct and two tail struct page structs) struct page
+ * of "corrupted mapping in tail page". We need to reset at least 4 (one
+ * head struct page struct and three tail struct page structs) struct page
   * structs.
   */
-#define NR_RESET_STRUCT_PAGE           3
+#define NR_RESET_STRUCT_PAGE           4
  
  static inline void reset_struct_pages(struct page *start)
  {

-- 
Cheers,

David / dhildenb