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

[Ryan Roberts]

On 18/03/2024 10:05, David Hildenbrand wrote:
> On 18.03.24 11:00, Yin, Fengwei wrote:
>>
>>
>> On 3/18/2024 10:16 AM, Huang, Ying wrote:
>>> 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?
>> I don't think READ_ONCE() can replace the lock.

But it doesn't ensure we get a consistent value and that the compiler orders the
load correctly. There are lots of patterns in the kernel that use READ_ONCE()
without a lock and they don't use data_race() - e.g. ptep_get_lockless().

It sounds like none of us really understand what data_race() is for, so I guess
I'll just do a KCSAN build and invoke the code path to see if it complains.


>>
>>>>>
>>>>> 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?
>> 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.
>