Re: [PATCH v3 mm-new] fs/proc/page: avoid anon folio checks on typed pages except hugetlb

[David Hildenbrand <david@redhat.com>]

On 15.07.25 15:30, Harry Yoo wrote:
> On Tue, Jul 15, 2025 at 02:31:45PM +0200, David Hildenbrand wrote:
>> On 15.07.25 14:07, Harry Yoo wrote:
>>> folio_test_anon() and folio_test_ksm() may return false positives when
>>> invoked on typed pages (except hugetlb folios), because lower bits of
>>> folio->mapping may be set even when they do not indicate
>>> FOLIO_MAPPING_* flags.
>>>
>>> This leads to tools/mm/page-types reporting pages with
>>> KPF_SLAB, KPF_ANON and KPF_KSM (with flags, page-counts, MB omitted):
>>>     $ sudo ./page-types | grep slab
>>>     _______S___________________________________   slab
>>>     _______S____a________x_____________________   slab,anonymous,ksm
>>>
>>> Currently, and going forward (The New York interpretation),
>>> typed pages except hugetlb do not have FOLIO_MAPPING_* flags.
>>> In the future, they won't even cast to folios.
>>>
>>> For now, avoid checking FOLIO_MAPPING_* flags on pages if they are
>>> typed pages that are not hugetlb folios.
>>>
>>> Update the comment in FOLIO_MAPPING_* flags accordingly.
>>>
>>> Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head")
>>> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
>>> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
>>> ---
>>>
>>> v2->v3: Did not introduce folio_has_mapcount() per David's suggestion
>>> v2: https://lore.kernel.org/linux-mm/20250707120740.4413-1-harry.yoo@oracle.com
>>>
>>>    fs/proc/page.c             | 19 +++++++++++--------
>>>    include/linux/page-flags.h | 15 +++++++++------
>>>    2 files changed, 20 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/fs/proc/page.c b/fs/proc/page.c
>>> index 0cdc78c0d23f..07582dbdef45 100644
>>> --- a/fs/proc/page.c
>>> +++ b/fs/proc/page.c
>>> @@ -148,18 +148,21 @@ u64 stable_page_flags(const struct page *page)
>>>    	folio = page_folio(page);
>>>    	k = folio->flags;
>>> -	mapping = (unsigned long)folio->mapping;
>>> -	is_anon = mapping & FOLIO_MAPPING_ANON;
>>>    	/*
>>>    	 * pseudo flags for the well known (anonymous) memory mapped pages
>>>    	 */
>>> -	if (page_mapped(page))
>>> -		u |= 1 << KPF_MMAP;
>>> -	if (is_anon) {
>>> -		u |= 1 << KPF_ANON;
>>> -		if (mapping & FOLIO_MAPPING_KSM)
>>> -			u |= 1 << KPF_KSM;
>>> +	if (!page_has_type(&folio->page) || folio_test_hugetlb(folio)) {
>>> +		mapping = (unsigned long)folio->mapping;
>>> +		is_anon = mapping & FOLIO_MAPPING_ANON;
>>> +
>>> +		if (page_mapped(page))
>>> +			u |= 1 << KPF_MMAP;
>>
>> Note: Luiz switches to folio_mapped() in his patch that is in mm-new.
> 
> Right, but in mm-new Luiz's patch already depends on my v2 patch,
> which still leaves me confused about what I should do... :)

Haha, right. Yes, just send it as a replacement to you current patch 
then. The conflict should be easy to resolve.

-- 
Cheers,

David / dhildenb