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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/18/24 18:05, David Hildenbrand wrote: > On 18.03.24 11:00, Yin, Fengwei wrote: >> >> >> On 3/18/2024 10:16 AM, Huang, Ying wrote: >>> Ryan Roberts 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 >>>>>>>> --- >>>>>>>>      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? No idea why there is no data_race() added. But I think we should add data_race(). Regards Yin, Fengwei > > It's used a bit inconsistently here. >