Re: [PATCH v2 2/2] mm: collect the number of anon large folios on split_deferred list

[David Hildenbrand <david@redhat.com>]

On 22.08.24 00:01, Barry Song wrote:
> On Thu, Aug 22, 2024 at 5:39 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 12.08.24 00:49, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> When an mTHP is added to the deferred_list, its partial pages
>>> are unused, leading to wasted memory and potentially increasing
>>> memory reclamation pressure.
>>>
>>> Detailing the specifics of how unmapping occurs is quite difficult
>>> and not that useful, so we adopt a simple approach: each time an
>>> mTHP enters the deferred_list, we increment the count by 1; whenever
>>> it leaves for any reason, we decrement the count by 1.
>>>
>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>> ---
>>>    Documentation/admin-guide/mm/transhuge.rst | 5 +++++
>>>    include/linux/huge_mm.h                    | 1 +
>>>    mm/huge_memory.c                           | 6 ++++++
>>>    3 files changed, 12 insertions(+)
>>>
>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>>> index 9fdfb46e4560..7072469de8a8 100644
>>> --- a/Documentation/admin-guide/mm/transhuge.rst
>>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>>> @@ -532,6 +532,11 @@ nr_anon
>>>           These huge pages could be entirely mapped or have partially
>>>           unmapped/unused subpages.
>>>
>>> +nr_split_deferred
>>> +       the number of anon huge pages which have been partially unmapped
>>> +       and put onto split queue. Those unmapped subpages are also unused
>>> +       and temporarily wasting memory.
>>
>> The name suggests something else ... like a counter of how many have
>> been deferred split :)
>>
>> Would "nr_anon_partially_mapped" "nr_anon_split_pending" (or something
>> less mouthful) be clearer?
>>
>> (likely "anon" really should be part of the name in any case)
>>
>> The name we chose (and the implied semantics) will likely have
>> implications on the handling of Usamas series.
>>
> 
> Hi David,
> 
> Your point is quite similar to my V1, though not exactly the same. I aimed to
> make the name more meaningful for users.
> https://lore.kernel.org/all/20240808010457.228753-3-21cnbao@gmail.com/
> 
> Ryan felt that the name should be consistent with the existing split_deferred.
> https://lore.kernel.org/all/36e8f1be-868d-4bce-8f32-e2d96b8b7af3@arm.com/#t

Right, it's an increasing counter of how often we added something to the 
deferred split queue. It's more like a low-level primitive, although 
currently only used for one reason (anon + partial mapping).

> 
> It seems that the existing split_deferred may now be less optimal with
> Usama's series, as entirely_mapped folios might also be on the list.

Yes.

> 
> Ryan is out right now, but I suppose he will be convinced that
> "nr_anon_partially_mapped" is probably a better name once
> he returns and reviews Usama's series. :-)

Maybe ;) At least it's clear from the name what we are talking about.

-- 
Cheers,

David / dhildenb