Re: [PATCH] mm/slab: fix folio_test_{anon,ksm}() false positive on slabs

[David Hildenbrand <david@redhat.com>]

On 10.06.25 15:17, Harry Yoo wrote:
> On Tue, Jun 10, 2025 at 03:03:12PM +0200, Vlastimil Babka wrote:
>> On 6/10/25 14:38, Harry Yoo wrote:
>>> On Mon, Jun 09, 2025 at 04:22:01PM +0200, Vlastimil Babka wrote:
>>>>> With the change, the nonsense slab pages disappear:
>>>>>
>>>>>    $ sudo ./page-types | grep slab
>>>>>    _______S___________________________________	slab
>>>>>
>>>>> Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head")
>>>>> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
>>>>> ---
>>>>>
>>>>> No Cc: stable because we don't usually check folio_test_anon() on slabs.
>>>>
>>>> Hmm maybe we could just make the code generating /proc/kpageflags stop
>>>> testing/reporting those "flags" for slab pages?
>>>>
>>>> It could be even more future proof code for when struct slab is separated.
>>>
>>> Not sure if I follow. When struct slab is separated, checking if a memdesc
>>> refers to slab or not will not have false positives, and thus no need for
>>> "must check folio_test_slab() before checking folio_test_anon()" rule?
>>
>> Not sure I follow either. I mean today we can add code testing
>> folio_test_slab() and only evaluating the mapping flags (ksm, anon) if it's
>> not slab. Once we convert to memdescs, that code will stay logically the
>> same even the testing functions will be named differently, no?
> 
> Ah, I thought you were saying it is unnecessary to fix folio_test_slab()
> false positives as long as we always check folio_test_slab() before
> folio_test_anon().

Yes, relevant code should do that. And we already do it at a couple of 
places (e.g., folio_expected_ref_count()).

-- 
Cheers,

David / dhildenb