linux-kernel - Re: [PATCH v5 5/6] mm: vmscan: Avoid split during shrink_folio

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Message-ID: <47fa11db-3c26-48a6-bb7e-5ea59739b691@arm.com>
Date: Tue, 2 Apr 2024 14:22:44 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>,
 Huang Ying <ying.huang@...el.com>, Gao Xiang <xiang@...nel.org>,
 Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
 Michal Hocko <mhocko@...e.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 Chris Li <chrisl@...nel.org>, Lance Yang <ioworker0@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v5 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 02/04/2024 14:10, Ryan Roberts wrote:
> On 28/03/2024 08:18, Barry Song wrote:
>> On Thu, Mar 28, 2024 at 3:45 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>>
>>> Now that swap supports storing all mTHP sizes, avoid splitting large
>>> folios before swap-out. This benefits performance of the swap-out path
>>> by eliding split_folio_to_list(), which is expensive, and also sets us
>>> up for swapping in large folios in a future series.
>>>
>>> If the folio is partially mapped, we continue to split it since we want
>>> to avoid the extra IO overhead and storage of writing out pages
>>> uneccessarily.
>>>
>>> Reviewed-by: David Hildenbrand <david@...hat.com>
>>> Reviewed-by: Barry Song <v-songbaohua@...o.com>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>> ---
>>>  mm/vmscan.c | 9 +++++----
>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 00adaf1cb2c3..293120fe54f3 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -1223,11 +1223,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>>                                         if (!can_split_folio(folio, NULL))
>>>                                                 goto activate_locked;
>>>                                         /*
>>> -                                        * Split folios without a PMD map right
>>> -                                        * away. Chances are some or all of the
>>> -                                        * tail pages can be freed without IO.
>>> +                                        * Split partially mapped folios right
>>> +                                        * away. We can free the unmapped pages
>>> +                                        * without IO.
>>>                                          */
>>> -                                       if (!folio_entire_mapcount(folio) &&
>>> +                                       if (data_race(!list_empty(
>>> +                                               &folio->_deferred_list)) &&
>>>                                             split_folio_to_list(folio,
>>>                                                                 folio_list))
>>>                                                 goto activate_locked;
>>
>> Hi Ryan,
>>
>> Sorry for bringing up another minor issue at this late stage.
> 
> No problem - I'd rather take a bit longer and get it right, rather than rush it
> and get it wrong!
> 
>>
>> During the debugging of thp counter patch v2, I noticed the discrepancy between
>> THP_SWPOUT_FALLBACK and THP_SWPOUT.
>>
>> Should we make adjustments to the counter?
> 
> Yes, agreed; we want to be consistent here with all the other existing THP
> counters; they only refer to PMD-sized THP. I'll make the change for the next
> version.
> 
> I guess we will eventually want equivalent counters for per-size mTHP using the
> framework you are adding.
> 
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 293120fe54f3..d7856603f689 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1241,8 +1241,10 @@ static unsigned int shrink_folio_list(struct
>> list_head *folio_list,
>>                                                                 folio_list))
>>                                                 goto activate_locked;
>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -
>> count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
>> -                                       count_vm_event(THP_SWPOUT_FALLBACK);
>> +                                       if (folio_test_pmd_mappable(folio)) {

This doesn't quite work because we have already split the folio here, so this
will always return false. I've changed it to:

if (nr_pages >= HPAGE_PMD_NR) {


>> +
>> count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
>> +
>> count_vm_event(THP_SWPOUT_FALLBACK);
>> +                                       }
>>  #endif
>>                                         if (!add_to_swap(folio))
>>                                                 goto activate_locked_split;
>>
>>
>> Because THP_SWPOUT is only for pmd:
>>
>> static inline void count_swpout_vm_event(struct folio *folio)
>> {
>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>         if (unlikely(folio_test_pmd_mappable(folio))) {
>>                 count_memcg_folio_events(folio, THP_SWPOUT, 1);
>>                 count_vm_event(THP_SWPOUT);
>>         }
>> #endif
>>         count_vm_events(PSWPOUT, folio_nr_pages(folio));
>> }
>>
>> I can provide per-order counters for this in my THP counter patch.
>>
>>> --
>>> 2.25.1
>>>
>>
>> Thanks
>> Barry
>