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

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Message-ID: <b084270d-6c7a-400c-8f4b-0797d2ae5fa6@redhat.com>
Date: Mon, 18 Mar 2024 11:05:16 +0100
From: David Hildenbrand <david@...hat.com>
To: "Yin, Fengwei" <fengwei.yin@...el.com>, "Huang, Ying"
 <ying.huang@...el.com>, Ryan Roberts <ryan.roberts@....com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, 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>
Subject: Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 18.03.24 11:00, Yin, Fengwei wrote:
> 
> 
> On 3/18/2024 10:16 AM, Huang, Ying wrote:
>> Ryan Roberts <ryan.roberts@....com> writes:
>>
>>> Hi Yin Fengwei,
>>>
>>> On 15/03/2024 11:12, David Hildenbrand wrote:
>>>> On 15.03.24 11:49, Ryan Roberts wrote:
>>>>> 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?
> I don't think READ_ONCE() can replace the lock.
> 
>>>>
>>>> Yeah, there is only one use of data_race with that list.
>>>>
>>>> It was added in f3ebdf042df4 ("THP: avoid lock when check whether THP is in
>>>> deferred list").
>>>>
>>>> Looks like that was added right in v1 of that change [1], so my best guess is
>>>> that it is not actually required.
>>>>
>>>> If not required, likely we should just cleanup the single user.
>>>>
>>>> [1]
>>>> https://lore.kernel.org/linux-mm/20230417075643.3287513-2-fengwei.yin@intel.com/
>>>
>>> Do you have any recollection of why you added the data_race() markup?
>>
>> Per my understanding, this is used to mark that the code accesses
>> folio->_deferred_list without lock intentionally, while
>> folio->_deferred_list may be changed in parallel.  IIUC, this is what
>> data_race() is used for.  Or, my understanding is wrong?
> Yes. This is my understanding also.

Why don't we have a data_race() in deferred_split_folio() then, before 
taking the lock?

It's used a bit inconsistently here.

-- 
Cheers,

David / dhildenb