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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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 06.05.24 10:20, Barry Song wrote: > On Mon, May 6, 2024 at 8:06 PM David Hildenbrand wrote: >> >> On 04.05.24 01:40, Barry Song wrote: >>> On Fri, May 3, 2024 at 5:41 PM Ryan Roberts wrote: >>>> >>>> On 03/05/2024 01:50, Barry Song wrote: >>>>> From: Barry Song >>>>> >>>>> 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" >>>>> 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