Messages in this thread |  | | Date | Thu, 7 Mar 2024 11:54:22 +0100 | | Subject | Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free | | From | David Hildenbrand <> |
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On 07.03.24 11:50, Ryan Roberts wrote: > On 07/03/2024 09:33, Barry Song wrote: >> On Thu, Mar 7, 2024 at 10:07 PM Ryan Roberts <ryan.roberts@arm.com> wrote: >>> >>> On 07/03/2024 08:10, Barry Song wrote: >>>> On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@gmail.com> wrote: >>>>> >>>>> Hey Barry, >>>>> >>>>> Thanks for taking time to review! >>>>> >>>>> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@gmail.com> wrote: >>>>>> >>>>>> On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@gmail.com> wrote: >>>>>>> >>>>> [...] >>>>>>> +static inline bool can_mark_large_folio_lazyfree(unsigned long addr, >>>>>>> + struct folio *folio, pte_t *start_pte) >>>>>>> +{ >>>>>>> + int nr_pages = folio_nr_pages(folio); >>>>>>> + fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>>>>>> + >>>>>>> + for (int i = 0; i < nr_pages; i++) >>>>>>> + if (page_mapcount(folio_page(folio, i)) != 1) >>>>>>> + return false; >>>>>> >>>>>> we have moved to folio_estimated_sharers though it is not precise, so >>>>>> we don't do >>>>>> this check with lots of loops and depending on the subpage's mapcount. >>>>> >>>>> If we don't check the subpage’s mapcount, and there is a cow folio associated >>>>> with this folio and the cow folio has smaller size than this folio, >>>>> should we still >>>>> mark this folio as lazyfree? >>>> >>>> I agree, this is true. However, we've somehow accepted the fact that >>>> folio_likely_mapped_shared >>>> can result in false negatives or false positives to balance the >>>> overhead. So I really don't know :-) >>>> >>>> Maybe David and Vishal can give some comments here. >>>> >>>>> >>>>>> BTW, do we need to rebase our work against David's changes[1]? >>>>>> [1] https://lore.kernel.org/linux-mm/20240227201548.857831-1-david@redhat.com/ >>>>> >>>>> Yes, we should rebase our work against David’s changes. >>>>> >>>>>> >>>>>>> + >>>>>>> + return nr_pages == folio_pte_batch(folio, addr, start_pte, >>>>>>> + ptep_get(start_pte), nr_pages, flags, NULL); >>>>>>> +} >>>>>>> + >>>>>>> static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >>>>>>> unsigned long end, struct mm_walk *walk) >>>>>>> >>>>>>> @@ -676,11 +690,45 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >>>>>>> */ >>>>>>> if (folio_test_large(folio)) { >>>>>>> int err; >>>>>>> + unsigned long next_addr, align; >>>>>>> >>>>>>> - if (folio_estimated_sharers(folio) != 1) >>>>>>> - break; >>>>>>> - if (!folio_trylock(folio)) >>>>>>> - break; >>>>>>> + if (folio_estimated_sharers(folio) != 1 || >>>>>>> + !folio_trylock(folio)) >>>>>>> + goto skip_large_folio; >>>>>> >>>>>> >>>>>> I don't think we can skip all the PTEs for nr_pages, as some of them might be >>>>>> pointing to other folios. >>>>>> >>>>>> for example, for a large folio with 16PTEs, you do MADV_DONTNEED(15-16), >>>>>> and write the memory of PTE15 and PTE16, you get page faults, thus PTE15 >>>>>> and PTE16 will point to two different small folios. We can only skip when we >>>>>> are sure nr_pages == folio_pte_batch() is sure. >>>>> >>>>> Agreed. Thanks for pointing that out. >>>>> >>>>>> >>>>>>> + >>>>>>> + align = folio_nr_pages(folio) * PAGE_SIZE; >>>>>>> + next_addr = ALIGN_DOWN(addr + align, align); >>>>>>> + >>>>>>> + /* >>>>>>> + * If we mark only the subpages as lazyfree, or >>>>>>> + * cannot mark the entire large folio as lazyfree, >>>>>>> + * then just split it. >>>>>>> + */ >>>>>>> + if (next_addr > end || next_addr - addr != align || >>>>>>> + !can_mark_large_folio_lazyfree(addr, folio, pte)) >>>>>>> + goto split_large_folio; >>>>>>> + >>>>>>> + /* >>>>>>> + * Avoid unnecessary folio splitting if the large >>>>>>> + * folio is entirely within the given range. >>>>>>> + */ >>>>>>> + folio_clear_dirty(folio); >>>>>>> + folio_unlock(folio); >>>>>>> + for (; addr != next_addr; pte++, addr += PAGE_SIZE) { >>>>>>> + ptent = ptep_get(pte); >>>>>>> + if (pte_young(ptent) || pte_dirty(ptent)) { >>>>>>> + ptent = ptep_get_and_clear_full( >>>>>>> + mm, addr, pte, tlb->fullmm); >>>>>>> + ptent = pte_mkold(ptent); >>>>>>> + ptent = pte_mkclean(ptent); >>>>>>> + set_pte_at(mm, addr, pte, ptent); >>>>>>> + tlb_remove_tlb_entry(tlb, pte, addr); >>>>>>> + } >>>>>> >>>>>> Can we do this in batches? for a CONT-PTE mapped large folio, you are unfolding >>>>>> and folding again. It seems quite expensive. >>> >>> I'm not convinced we should be doing this in batches. We want the initial >>> folio_pte_batch() to be as loose as possible regarding permissions so that we >>> reduce our chances of splitting folios to the min. (e.g. ignore SW bits like >>> soft dirty, etc). I think it might be possible that some PTEs are RO and other >>> RW too (e.g. due to cow - although with the current cow impl, probably not. But >>> its fragile to assume that). Anyway, if we do an initial batch that ignores all >> >> You are correct. I believe this scenario could indeed occur. For instance, >> if process A forks process B and then unmaps itself, leaving B as the >> sole process owning the large folio. The current wp_page_reuse() function >> will reuse PTE one by one while the specific subpage is written. > > Hmm - I thought it would only reuse if the total mapcount for the folio was 1. > And since it is a large folio with each page mapped once in proc B, I thought > every subpage write would cause a copy except the last one? I haven't looked at > the code for a while. But I had it in my head that this is an area we need to > improve for mTHP.
wp_page_reuse() will currently reuse a PTE part of a large folio only if a single PTE remains mapped (refcount == 0).
-- Cheers,
David / dhildenb
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