Messages in this thread | | | Date | Mon, 18 Mar 2024 18:00:19 +0800 | Subject | Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list() | From | "Yin, Fengwei" <> |
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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.
Regards Yin, Fengwei
> > -- > Best Regards, > Huang, Ying
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