Messages in this thread | | | Date | Mon, 18 Mar 2024 11:05:16 +0100 | Subject | Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list() | From | David Hildenbrand <> |
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On 18.03.24 11:00, Yin, Fengwei wrote: > > > On 3/18/2024 10:16 AM, Huang, Ying wrote: >> Ryan Roberts <ryan.roberts@arm.com> 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@arm.com> >>>>>>> --- >>>>>>> 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?
It's used a bit inconsistently here.
-- Cheers,
David / dhildenb
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