linux-kernel - Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio

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Date: Fri, 15 Mar 2024 10:49:17 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 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>, Barry Song <21cnbao@...il.com>,
 Chris Li <chrisl@...nel.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 15/03/2024 10:43, David Hildenbrand wrote:
> On 11.03.24 16:00, Ryan Roberts 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.
>>
>> 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 cf7d4cf47f1a..0ebec99e04c6 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1222,11 +1222,12 @@ 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 map
>> +                     * right away. Chances are some or all
>> +                     * of the tail pages can be freed
>> +                     * without IO.
>>                        */
>> -                    if (!folio_entire_mapcount(folio) &&
>> +                    if (!list_empty(&folio->_deferred_list) &&
>>                           split_folio_to_list(folio,
>>                                   folio_list))
>>                           goto activate_locked;
> 
> Not sure if we might have to annotate that with data_race().

I asked that exact question to Matthew in another context bt didn't get a
response. There are examples of checking if the deferred list is empty with and
without data_race() in the code base. But list_empty() is implemented like this:

static inline int list_empty(const struct list_head *head)
{
	return READ_ONCE(head->next) == head;
}

So I assumed the READ_ONCE() makes everything safe without a lock? Perhaps not
sufficient for KCSAN?


> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
> 

Thanks!