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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 375FE14001E X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: xcf4xscjcxnr8fudbws6st8rrizyuj6e X-HE-Tag: 1710776216-127447 X-HE-Meta: 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 3jkMCxn6 pEB8tE3g7sI7PSn4HaWM75LXqzpmvNMxxysNiqsZAww5M/wVyICn8gPHSrN727HSlpoSe4CqtZGmpPkKO08bdmt9NclTi88bd/lE2NlkWYXtMtlJF4n3QuBDb+KNXO5pwa8WL5/ED4Vi5Km1SO+tuQYYLZ06iP3yqDAM0SCY7lGO0Z0AiPImBuiLpbzwZqd85Q7eN7wRUuYv5VKOoFCLC/IxO1TVAssLPmYAxm2b+8//8yzvFLTABrhkIw9KmNhEUMTRXvnB9hsp8ETzEuDPLfxP3gAsT4XOg3k7yytPwFfMOD/zUPcRCGH7zVr64JfuwsLmoi4Zg4aAMwtgpYpdsi+NHMYI+nKbfg/zC9DJuvo11gH8MA3enULXN0/fIPS5igIWK2jSPCXdBxlTVQvf46AzpoQ== 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 18/03/2024 15:35, Ryan Roberts wrote: > 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 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. > > But it doesn't ensure we get a consistent value and that the compiler orders the Sorry - fat fingers... I meant it *does* ensure we get a consistent value (i.e. untorn) > 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@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. >> >