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

[Vlastimil Babka <vbabka@suse.cz>]

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?

> Of course, to fix false positive completely, I think we need to add
> something like:
> 
> __aligned(4) or __aligned(CONFIG_FUNCTION_ALIGNMENT) (whichever alignment is bigger)
> 
> to rcu_free_slab().

OK.

>> I mean there's even a comment above PAGE_MAPPING_ANON:
>>
>>  * For slab pages, since slab reuses the bits in struct page to store its
>>  * internal states, the folio->mapping does not exist as such, nor do
>>  * these flags below.  So in order to avoid testing non-existent bits,
>>  * please make sure that folio_test_slab(folio) actually evaluates to
>>  * false before calling the following functions (e.g., folio_test_anon).
>>  * See mm/slab.h.
> 
> That comment was added because it was a hard lesson learned from
> false positive. 
>