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? >>> 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. >
