Re: [PATCH] mm/huge_memory: use folio_dup_file_rmap_pud() in copy_huge_pud()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 11/7/26 18:05, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found no new issues
> in this patch, but noticed 1 related pre-existing issue it does not
> introduce:
> - [Medium] mm: hardware A/D bits discarded for PUD folios in
>   zap_huge_pud() [1]
> --
> 
> commit a6bb79712598f2da7fdcc94c9b1b5f8b9aad8851
> Author: Konstantin Khorenko <khorenko@virtuozzo.com>
> 
> mm/huge_memory: use folio_dup_file_rmap_pud() in copy_huge_pud()
> 
> This commit adds folio_dup_file_rmap_pud() to duplicate file rmap mapcounts
> at the PUD level and uses it in copy_huge_pud() instead of
> folio_add_file_rmap_pud().
> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index d0e19d9e6b651..13caf81cb51af 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2100,7 +2100,7 @@ int copy_huge_pud(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>>  		 * TODO: once we support anonymous pages, use
>>  		 * folio_try_dup_anon_rmap_*() and split if duplicating fails.
>>  		 */
>> -		folio_add_file_rmap_pud(folio, page, dst_vma);
>> +		folio_dup_file_rmap_pud(folio, page, dst_vma);
>>  		add_mm_counter(dst_mm, mm_counter_file(folio), HPAGE_PUD_NR);
>>  	}
> 
> [Severity: Medium]
> This problem wasn't introduced by this patch, but does zap_huge_pud()
> discard hardware accessed and dirty bits on the PUD folio [1]?
> 
> mm/huge_memory.c:zap_huge_pud() {
> 	...
> 	orig_pud = pudp_huge_get_and_clear_full(vma, addr, pud, tlb->fullmm);
> 	...
> 	} else {
> 		...
> 		page = pud_page(orig_pud);
> 		folio = page_folio(page);
> 		folio_remove_rmap_pud(folio, page, vma);
> 		add_mm_counter(tlb->mm, mm_counter_file(folio), -HPAGE_PUD_NR);
> 	...
> }
> 
> Unlike zap_huge_pmd(), which checks the cleared entry and calls
> folio_mark_accessed() and folio_mark_dirty(), zap_huge_pud() removes the
> PUD rmap without propagating the young and dirty bits from orig_pud to the
> folio.  When unmapping a refcounted Device DAX PUD mapping, those hardware
> A/D bits are lost, leaving the folio with stale access and dirty state.

Probably not that relevant for fsdax, which is the only one supporting PUD THPs
IIRC.

-- 
Cheers,

David