Re: [PATCH v2 1/5] mm: Separate folio_split_memcg_refs() from split_page_memcg()

[Zi Yan <ziy@nvidia.com>]

On 14 Mar 2025, at 17:49, David Hildenbrand wrote:

> On 14.03.25 14:36, Matthew Wilcox (Oracle) wrote:
>> Folios always use memcg_data to refer to the mem_cgroup while pages
>> allocated with GFP_ACCOUNT have a pointer to the obj_cgroup.  Since the
>> caller already knows what it has, split the function into two and then
>> we don't need to check.
>>
>> Move the assignment of split folio memcg_data to the point where we set
>> up the other parts of the new folio.  That leaves folio_split_memcg_refs()
>> just handling the memcg accounting.
>>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   include/linux/memcontrol.h |  7 +++++++
>>   mm/huge_memory.c           | 16 ++++------------
>>   mm/memcontrol.c            | 17 +++++++++++++----
>>   3 files changed, 24 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 57664e2a8fb7..d090089c5497 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -1039,6 +1039,8 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
>>   }
>>    void split_page_memcg(struct page *head, int old_order, int new_order);
>> +void folio_split_memcg_refs(struct folio *folio, unsigned old_order,
>> +		unsigned new_order);
>>    static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
>>   {
>> @@ -1463,6 +1465,11 @@ static inline void split_page_memcg(struct page *head, int old_order, int new_or
>>   {
>>   }
>>  +static inline void folio_split_memcg_refs(struct folio *folio,
>> +		unsigned old_order, unsigned new_order)
>> +{
>> +}
>> +
>>   static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
>>   {
>>   	return 0;
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 14b1963898a7..3e5ecc8f3d13 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3394,6 +3394,9 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>>   			folio_set_young(new_folio);
>>   		if (folio_test_idle(folio))
>>   			folio_set_idle(new_folio);
>> +#ifdef CONFIG_MEMCG
>> +		new_folio->memcg_data = folio->memcg_data;
>> +#endif
>>    		folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio));
>>   	}
>> @@ -3525,18 +3528,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>   			}
>>   		}
>>  -		/*
>> -		 * Reset any memcg data overlay in the tail pages.
>> -		 * folio_nr_pages() is unreliable until prep_compound_page()
>> -		 * was called again.
>> -		 */
>> -#ifdef NR_PAGES_IN_LARGE_FOLIO
>> -		folio->_nr_pages = 0;
>> -#endif
>
>
> I remember that we could trigger a warning without that, but I don't immediately find where that warning was. IIRC, if we'd split to order-0, page[1] would have indicated that it had a memcg set, and something bailed out.
>
> Maybe Zi Yan recalls where that check fired.

The error I encountered is different. When I rebase my folio_split()
on top of David’s mapcount patchset, my original patch used folio_nr_pages()
after memcg split. Since memcg overlays with _nr_pages, when splitting
to order-0, folio->_nr_page is overwritten with memcg_data, causing
folio_nr_pages() to return a bogus value. With Matthew’s this patch,
memcg_data of page[1] is written inside __split_folio_to_order(),
so in theory __split_folio_to_order() can call folio_nr_pages() like
my original patch.

For folio->_nr_pages = 0, I suppose it is trying to suppress any
page[1]->memcg_data != NULL check in the following code. But I could
not find any.

>
> In any case, if that warning no longer fires this is a very nice cleanup!

Yeah, if we see any warning on memcg later, we know how to fix it. :)


Best Regards,
Yan, Zi