Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx>
>>>>> ---
>>>>>    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?
>> 
>> 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@xxxxxxxxx/
>
> 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?

--
Best Regards,
Huang, Ying