Re: [PATCH] mm/thp: expose deferred split folio memory usage in meminfo

[Ye Liu]

在 2026/7/17 17:48, Lorenzo Stoakes (ARM) 写道:
> On Fri, Jul 17, 2026 at 11:33:05AM +0200, David Hildenbrand (Arm) wrote:
>> On 7/17/26 11:31, Ye Liu wrote:
>>>
>>>
>>> 在 2026/7/17 16:37, David Hildenbrand (Arm) 写道:
>>>> On 7/17/26 08:30, Ye Liu wrote:
>>>>> From: Ye Liu <liuye@xxxxxxxxxx>
>>>>>
>>>>> Folios on the deferred split list hold physical memory that is
>>>>> invisible in meminfo. When a THP becomes partially mapped, the
>>>>> unmapped pages are removed from AnonPages but remain physically
>>>>> allocated until the shrinker splits the folio. This creates a
>>>>> memory accounting gap where used memory cannot be attributed to
>>>>> any meminfo field.
>>>>>
>>>>> Add NR_DEFERRED_SPLIT_PAGES to track the total memory consumed by
>>>>> folios currently on the deferred_split_lru, updated via
>>>>> mod_node_page_state() at all enqueue/dequeue points. The new field
>>>>> DeferredSplitPages is visible in /proc/meminfo, /proc/vmstat, and
>>>>> per-node /sys/devices/system/node/node*/meminfo.
>>>>>
>>>>
>>>> Oh no.
>>>>
>>>> This is really exposing a current implementation detail where we have this
>>>> information easily available in a way that we will not be able to change that
>>>> implementation later.
>>>>
>>>
>>> Yes, `/proc/meminfo` is a user-space ABI.
>>> Adding it there means you can't change the semantics,
>>> and the field will become unmaintainable, yet you can't delete it either.
>>> But I think this information is still necessary.
>>> Where would be a better place to put it?
>>
>> Don't expose it to user space.
> 
> Yeah please stop.
> 
> We don't want this change, it exposes internal implementation details. It's a
> no.
> 
> Next time send an RFC or a discussion thread.
> 
Yes, sending an RFC would be more appropriate.                                                  
                                                                                                
I'd also like to ask if it's possible to implement this in                                      
`/sys/kernel/debug/deferred_split_pages`, or are you completely against querying this statistic?
                                                                                                
How can we quickly identify this situation?                                                     
I believe some people will still be confused: where did the memory go?                          

>>
>> --
>> Cheers,
>>
>> David
> 
> Thanks, Lorenzo

-- 
Thanks,
Ye Liu