Re: [mm/sl[au]b] 3c4cafa313: canonical_address#:#[##]

Vlastimil Babka
2022-09-16T17:08:00Z

On 9/14/22 09:42, Matthew Wilcox wrote:
> On Wed, Sep 14, 2022 at 03:33:50PM +0900, Hyeonggon Yoo wrote:
>> On Fri, Sep 09, 2022 at 11:16:51PM +0200, Vlastimil Babka wrote:
>>> On 9/9/22 16:32, Hyeonggon Yoo wrote:
>>>> On Fri, Sep 09, 2022 at 03:44:19PM +0200, Vlastimil Babka wrote:
>>>>> On 9/9/22 13:05, Hyeonggon Yoo wrote:
>>>>>>> ----8<----
>>>>>>> From d6f9fbb33b908eb8162cc1f6ce7f7c970d0f285f Mon Sep 17 00:00:00 2001
>>>>>>> From: Vlastimil Babka <vbabka@suse.cz>
>>>>>>> Date: Fri, 9 Sep 2022 12:03:10 +0200
>>>>>>> Subject: [PATCH 2/3] mm/migrate: make isolate_movable_page() skip slab pages
>>>>>>>
>>>>>>> In the next commit we want to rearrange struct slab fields to allow a
>>>>>>> larger rcu_head. Afterwards, the page->mapping field will overlap
>>>>>>> with SLUB's "struct list_head slab_list", where the value of prev
>>>>>>> pointer can become LIST_POISON2, which is 0x122 + POISON_POINTER_DELTA.
>>>>>>> Unfortunately the bit 1 being set can confuse PageMovable() to be a
>>>>>>> false positive and cause a GPF as reported by lkp [1].
>>>>>>>
>>>>>>> To fix this, make isolate_movable_page() skip pages with the PageSlab
>>>>>>> flag set. This is a bit tricky as we need to add memory barriers to SLAB
>>>>>>> and SLUB's page allocation and freeing, and their counterparts to
>>>>>>> isolate_movable_page().
>>>>>>
>>>>>> Hello, I just took a quick grasp,
>>>>>> Is this approach okay with folio_test_anon()?
>>>>>
>>>>> Not if used on a completely random page as compaction scanners can, but
>>>>> relies on those being first tested for PageLRU or coming from a page table
>>>>> lookup etc.
>>>>> Not ideal huh. Well I could improve also by switching 'next' and 'slabs'
>>>>> field and relying on the fact that the value of LIST_POISON2 doesn't include
>>>>> 0x1, just 0x2.
>>>>
>>>> What about swapping counters and freelist?
>>>> freelist should be always aligned.
>>>
>>> Great suggestion, thanks!
>>>
>>> Had to deal with SLAB too as there was list_head.prev also aliasing
>>> page->mapping. Wanted to use freelist as well, but turns out it's not
>>> aligned, so had to use s_mem instead.
>>>
>>> The patch that isolate_movable_page() skip slab pages was thus dropped. The
>>> result is in slab.git below and if nothing blows up, will restore it to -next
>>>
>>> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=for-6.1/fit_rcu_head
>>
>> I realized that there is also relevant comment in
>> include/linux/mm_types.h:
>>
>>> 62 * SLUB uses cmpxchg_double() to atomically update its freelist and counters.
>>> 63 * That requires that freelist & counters in struct slab be adjacent and
>>> 64 * double-word aligned. Because struct slab currently just reinterprets the
>>> 65 * bits of struct page, we align all struct pages to double-word boundaries,
>>> 66 * and ensure that 'freelist' is aligned within struct slab.
>>> 67 */
>>
>> Also we may add a comment,
>> something like this?
>>
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -79,6 +79,9 @@ struct page {
>> * WARNING: bit 0 of the first word is used for PageTail(). That
>> * means the other users of this union MUST NOT use the bit to
>> * avoid collision and false-positive PageTail().
>> + *
>> + * WARNING: lower two bits of third word is used for PAGE_MAPPING_FLAGS.
>> + * using those bits can lead compaction code to general protection fault.
>
> I'm really not comfortable with adding that documentation. I feel the
> compaction code should be fixed.

Any suggestions how exactly, as using a true page flag for __PageMovable
is off limits? :)