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[2003:cb:c74b:bf00:182c:d606:87cf:6fea]) by smtp.gmail.com with ESMTPSA id u17-20020a05600c19d100b0041bb11ff5a7sm19042298wmq.8.2024.05.06.01.31.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 May 2024 01:31:22 -0700 (PDT) Message-ID: Date: Mon, 6 May 2024 10:31:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/6] mm: introduce pte_move_swp_offset() helper which can move offset bidirectionally To: Barry Song <21cnbao@gmail.com> Cc: Ryan Roberts , akpm@linux-foundation.org, linux-mm@kvack.org, baolin.wang@linux.alibaba.com, chrisl@kernel.org, hanchuanhua@oppo.com, hannes@cmpxchg.org, hughd@google.com, kasong@tencent.com, linux-kernel@vger.kernel.org, surenb@google.com, v-songbaohua@oppo.com, willy@infradead.org, xiang@kernel.org, ying.huang@intel.com, yosryahmed@google.com, yuzhao@google.com, ziy@nvidia.com References: <20240503005023.174597-1-21cnbao@gmail.com> <20240503005023.174597-4-21cnbao@gmail.com> <7548e30c-d56a-4a57-ab87-86c9c8e523b1@arm.com> <0d20d8af-e480-4eb8-8606-1e486b13fd7e@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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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" >>>>> Signed-off-by: Barry Song >>>>> --- >>>>> 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@redhat.com/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. 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) >> > > 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. -- Cheers, David / dhildenb