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[2003:cb:c739:2400:78ac:64bb:a39e:2578]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4256b068e93sm198233215e9.24.2024.07.02.06.01.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Jul 2024 06:01:06 -0700 (PDT) Message-ID: <0b6d36b9-ce12-4278-a697-0186088e17b1@redhat.com> Date: Tue, 2 Jul 2024 15:01:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages To: Alistair Popple References: <874j98gjfg.fsf@nvdebian.thelocal> <0b549ff0-b0b6-4fc8-aa6f-0d76157575b3@redhat.com> <87wmm4f1y6.fsf@nvdebian.thelocal> From: David Hildenbrand 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: <87wmm4f1y6.fsf@nvdebian.thelocal> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linmiaohe@huawei.com, nvdimm@lists.linux.dev, jack@suse.cz, linux-doc@vger.kernel.org, djwong@kernel.org, dave.hansen@linux.intel.com, david@fromorbit.com, peterx@redhat.com, linux-mm@kvack.org, will@kernel.org, hch@lst.de, dave.jiang@intel.com, vishal.l.verma@intel.com, willy@infradead.org, jgg@ziepe.ca, catalin.marinas@arm.com, linux-ext4@vger.kernel.org, ira.weiny@intel.com, jhubbard@nvidia.com, npiggin@gmail.com, linux-cxl@vger.kernel.org, bhelgaas@google.com, dan.j.williams@intel.com, linux-arm-kernel@lists.infradead.org, tytso@mit.edu, logang@deltatee.com, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 02.07.24 13:30, Alistair Popple wrote: > > David Hildenbrand writes: > >> On 02.07.24 12:19, Alistair Popple wrote: >>> David Hildenbrand writes: >>> >>>> On 27.06.24 02:54, Alistair Popple wrote: >>>>> Currently DAX folio/page reference counts are managed differently to >>>>> normal pages. To allow these to be managed the same as normal pages >>>>> introduce dax_insert_pfn_pud. This will map the entire PUD-sized folio >>>>> and take references as it would for a normally mapped page. >>>>> This is distinct from the current mechanism, vmf_insert_pfn_pud, >>>>> which >>>>> simply inserts a special devmap PUD entry into the page table without >>>>> holding a reference to the page for the mapping. >>>> >>>> Do we really have to involve mapcounts/rmap for daxfs pages at this >>>> point? Or is this only "to make it look more like other pages" ? >>> The aim of the series is make FS DAX and other ZONE_DEVICE pages >>> look >>> like other pages, at least with regards to the way they are refcounted. >>> At the moment they are not refcounted - instead their refcounts are >>> basically statically initialised to one and there are all these special >>> cases and functions requiring magic PTE bits (pXX_devmap) to do the >>> special DAX reference counting. This then adds some cruft to manage >>> pgmap references and to catch the 2->1 page refcount transition. All >>> this just goes away if we manage the page references the same as other >>> pages (and indeed we already manage DEVICE_PRIVATE and COHERENT pages >>> the same as normal pages). >>> So I think to make this work we at least need the mapcounts. >>> >> >> We only really need the mapcounts if we intend to do something like >> folio_mapcount() == folio_ref_count() to detect unexpected folio >> references, and if we have to have things like folio_mapped() >> working. For now that was not required, that's why I am asking. > > Oh I see, thanks for pointing that out. In that case I agree, we don't > need the mapcounts. As you say we don't currently need to detect > unexpect references for FS DAX and this series doesn't seek to introduce > any new behviour/features. > >> Background also being that in a distant future folios will be >> decoupled more from other compound pages, and only folios (or "struct >> anon_folio" / "struct file_folio") would even have mapcounts. >> >> For example, most stuff we map (and refcount!) via vm_insert_page() >> really must stop involving mapcounts. These won't be "ordinary" >> mapcount-tracked folios in the future, they are simply some refcounted >> pages some ordinary driver allocated. > > Ok, so for FS DAX we should take a reference on the folio for the > mapping but not a mapcount? That's what we could do, yes. But if we really just want to track page table mappings like we do with other folios (anon/pagecache/shmem), and want to keep doing that in the future, then maybe we should just do it right away. The rmap code you add will be required in the future either way. Using that code for device dax would likely be impossible right now due to the vmemmap optimizations I guess, if we would end up writing subpage mapcounts. But for FSDAX purposes (no vmemmap optimization) it would work. -- Cheers, David / dhildenb