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[2003:cb:c739:2400:78ac:64bb:a39e:2578]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3678aa3caccsm676393f8f.35.2024.07.02.04.02.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Jul 2024 04:02:30 -0700 (PDT) Message-ID: <0b549ff0-b0b6-4fc8-aa6f-0d76157575b3@redhat.com> Date: Tue, 2 Jul 2024 13:02:28 +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> 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: <874j98gjfg.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 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. 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. For FS-DAX, if we'll be using the same "struct file_folio" approach as for ordinary pageache memory, then this is the right thing to do here. >> I'm asking this because: >> >> (A) We don't support mixing PUD+PMD mappings yet. I have plans to change >> that in the future, but for now you can only map using a single PUD >> or by PTEs. I suspect that's good enoug for now for dax fs? > > Yep, that's all we support. > >> (B) As long as we have subpage mapcounts, this prevents vmemmap >> optimizations [1]. Is that only used for device-dax for now and are >> there no plans to make use of that for fs-dax? > > I don't have any plans to. This is purely focussed on refcounting pages > "like normal" so we can get rid of all the DAX special casing. > >> (C) We managed without so far :) > > Indeed, although Christoph has asked repeatedly ([1], [2] and likely > others) that this gets fixed and I finally got sick of it coming up > everytime I need to touch something with ZONE_DEVICE pages :) > > Also it removes the need for people to understand the special DAX page > recounting scheme and ends up removing a bunch of cruft as a bonus: > > 59 files changed, 485 insertions(+), 869 deletions(-) I'm not challenging the refcounting scheme. I'm purely asking about mapcount handling, which is something related but different. > > And that's before I clean up all the pgmap reference handling. It also > removes the pXX_trans_huge and pXX_leaf distinction. So we managed, but > things could be better IMHO. > Again, all nice things. >> Having that said, with folio->_large_mapcount things like >> folio_mapcount() are no longer terribly slow once we weould PTE-map a >> PUD-sized folio. >> >> Also, all ZONE_DEVICE pages should currently be marked PG_reserved, >> translating to "don't touch the memmap". I think we might want to >> tackle that first. Missed to add a pointer to [2]. > > Ok. I'm keen to get this series finished and I don't quite get the > connection here, what needs to change there? include/linux/page-flags.h "PG_reserved is set for special pages. The "struct page" of such a page should in general not be touched (e.g. set dirty) except by its owner. Pages marked as PG_reserved include: ... - Device memory (e.g. PMEM, DAX, HMM) " I think we already entered that domain with other ZONE_DEVICE pages being returned from vm_normal_folio(), unfortunately. But that really must be cleaned up for these pages to not look special anymore. Agreed that it likely is something that is not blocking this series. -- Cheers, David / dhildenb