On 3/18/24 23:35, 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 <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(). They (ptep_get_lockless() and deferred_list) have different access pattern (or race pattern) here. I don't think they are comparable. > > 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. READ_ONCE() in list_empty will shutdown the KCSAN also. > > >>> >>>>>> >>>>>> 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. >> >