On 07/05/2024 09:24, Barry Song wrote: > On Tue, May 7, 2024 at 8:14 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >> >> On 06/05/2024 09:31, David Hildenbrand wrote: >>> On 06.05.24 10:20, Barry Song wrote: >>>> On Mon, May 6, 2024 at 8:06 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>>> >>>>> On 04.05.24 01:40, Barry Song wrote: >>>>>> On Fri, May 3, 2024 at 5:41 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >>>>>>> >>>>>>> On 03/05/2024 01:50, Barry Song wrote: >>>>>>>> From: Barry Song <v-songbaohua@xxxxxxxx> >>>>>>>> >>>>>>>> There could arise a necessity to obtain the first pte_t from a swap >>>>>>>> pte_t located in the middle. For instance, this may occur within the >>>>>>>> context of do_swap_page(), where a page fault can potentially occur in >>>>>>>> any PTE of a large folio. To address this, the following patch introduces >>>>>>>> pte_move_swp_offset(), a function capable of bidirectional movement by >>>>>>>> a specified delta argument. Consequently, pte_increment_swp_offset() >>>>>>> >>>>>>> You mean pte_next_swp_offset()? >>>>>> >>>>>> yes. >>>>>> >>>>>>> >>>>>>>> will directly invoke it with delta = 1. >>>>>>>> >>>>>>>> Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx> >>>>>>>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx> >>>>>>>> --- >>>>>>>> mm/internal.h | 25 +++++++++++++++++++++---- >>>>>>>> 1 file changed, 21 insertions(+), 4 deletions(-) >>>>>>>> >>>>>>>> diff --git a/mm/internal.h b/mm/internal.h >>>>>>>> index c5552d35d995..cfe4aed66a5c 100644 >>>>>>>> --- a/mm/internal.h >>>>>>>> +++ b/mm/internal.h >>>>>>>> @@ -211,18 +211,21 @@ static inline int folio_pte_batch(struct folio >>>>>>>> *folio, unsigned long addr, >>>>>>>> } >>>>>>>> >>>>>>>> /** >>>>>>>> - * pte_next_swp_offset - Increment the swap entry offset field of a swap >>>>>>>> pte. >>>>>>>> + * pte_move_swp_offset - Move the swap entry offset field of a swap pte >>>>>>>> + * forward or backward by delta >>>>>>>> * @pte: The initial pte state; is_swap_pte(pte) must be true and >>>>>>>> * non_swap_entry() must be false. >>>>>>>> + * @delta: The direction and the offset we are moving; forward if delta >>>>>>>> + * is positive; backward if delta is negative >>>>>>>> * >>>>>>>> - * Increments the swap offset, while maintaining all other fields, including >>>>>>>> + * Moves the swap offset, while maintaining all other fields, including >>>>>>>> * swap type, and any swp pte bits. The resulting pte is returned. >>>>>>>> */ >>>>>>>> -static inline pte_t pte_next_swp_offset(pte_t pte) >>>>>>>> +static inline pte_t pte_move_swp_offset(pte_t pte, long delta) >>>>>>> >>>>>>> We have equivalent functions for pfn: >>>>>>> >>>>>>> pte_next_pfn() >>>>>>> pte_advance_pfn() >>>>>>> >>>>>>> Although the latter takes an unsigned long and only moves forward currently. I >>>>>>> wonder if it makes sense to have their naming and semantics match? i.e. change >>>>>>> pte_advance_pfn() to pte_move_pfn() and let it move backwards too. >>>>>>> >>>>>>> I guess we don't have a need for that and it adds more churn. >>>>>> >>>>>> we might have a need in the below case. >>>>>> A forks B, then A and B share large folios. B unmap/exit, then large >>>>>> folios of process >>>>>> A become single-mapped. >>>>>> Right now, while writing A's folios, we are CoWing A's large folios >>>>>> into many small >>>>>> folios. I believe we can reuse the entire large folios instead of doing >>>>>> nr_pages >>>>>> CoW and page faults. >>>>>> In this case, we might want to get the first PTE from vmf->pte. >>>>> >>>>> Once we have COW reuse for large folios in place (I think you know that >>>>> I am working on that), it might make sense to "COW-reuse around", >>>> >>>> TBH, I don't know if you are working on that. please Cc me next time :-) >>> >>> I could have sworn I mentioned it to you already :) >>> >>> See >>> >>> https://lore.kernel.org/linux-mm/a9922f58-8129-4f15-b160-e0ace581bcbe@xxxxxxxxxx/T/ >>> >>> I'll follow-up on that soonish (now that batching is upstream and the large >>> mapcount is on its way upstream). >>> >>>> >>>>> meaning we look if some neighboring PTEs map the same large folio and >>>>> map them writable as well. But if it's really worth it, increasing page >>>>> fault latency, is to be decided separately. >>>> >>>> On the other hand, we eliminate latency for the remaining nr_pages - 1 PTEs. >>>> Perhaps we can discover a more cost-effective method to signify that a large >>>> folio is probably singly mapped? >>> >>> Yes, precisely what I am up to! >>> >>>> and only call "multi-PTEs" reuse while that >>>> condition is true in PF and avoid increasing latency always? >>> >>> I'm thinking along those lines: >>> >>> If we detect that it's exclusive, we can certainly mapped the current PTE >>> writable. Then, we can decide how much (and if) we want to fault-around writable >>> as an optimization. >>> >>> For smallish large folios, it might make sense to try faulting around most of >>> the folio. >>> >>> For large large folios (e.g., PTE-mapped 2MiB THP and bigger), we might not want >>> to fault around the whole thing -- especially if there is little benefit to be >>> had from contig-pte bits. >>> >>>> >>>>> >>>>> >>>>>> >>>>>> Another case, might be >>>>>> A forks B, and we write either A or B, we might CoW an entire large >>>>>> folios instead >>>>>> CoWing nr_pages small folios. >>>>>> >>>>>> case 1 seems more useful, I might have a go after some days. then we might >>>>>> see pte_move_pfn(). >>>>> pte_move_pfn() does sound odd to me. >> >> Yes, I agree the name is odd. pte_move_swp_offset() sounds similarly odd tbh. >> Perhaps just pte_advance_swp_offset() with a negative value is clearer about >> what its doing? >> > > I am not a native speaker. but dictionary says > > advance: > move forward in a purposeful way. > a forward movement. > > Now we are moving backward or forward :-) Sure, but if you pass a negative value then you are moving forwards by a negative amount ;-) Anyway, forget I said anything - its not important. > >>>>> It might not be required to >>>>> implement the optimization described above. (it's easier to simply read >>>>> another PTE, check if it maps the same large folio, and to batch from there) >> >> Yes agreed. >> >>>>> >>>> >>>> It appears that your proposal suggests potential reusability as follows: if we >>>> have a large folio containing 16 PTEs, you might consider reusing only 4 by >>>> examining PTEs "around" but not necessarily all 16 PTEs. please correct me >>>> if my understanding is wrong. >>>> >>>> Initially, my idea was to obtain the first PTE using pte_move_pfn() and then >>>> utilize folio_pte_batch() with the first PTE as arguments to ensure consistency >>>> in nr_pages, thus enabling complete reuse of the whole folio. >>> >>> Simply doing an vm_normal_folio(pte - X) == folio and then trying to batch from >>> there might be easier and cleaner. >>> >>
