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(p200300d82f438900f36413332a67d49e.dip0.t-ipconnect.de. [2003:d8:2f43:8900:f364:1333:2a67:d49e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4562e886104sm76981725e9.25.2025.07.18.03.54.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 18 Jul 2025 03:54:39 -0700 (PDT) Message-ID: <8292201e-0ca2-4a2a-b2a7-02391cbf7556@redhat.com> Date: Fri, 18 Jul 2025 12:54:38 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 5/9] mm/huge_memory: mark PMD mappings of the huge zero folio special To: Lorenzo Stoakes Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, xen-devel@lists.xenproject.org, linux-fsdevel@vger.kernel.org, nvdimm@lists.linux.dev, Andrew Morton , Juergen Gross , Stefano Stabellini , Oleksandr Tyshchenko , Dan Williams , Matthew Wilcox , Jan Kara , Alexander Viro , Christian Brauner , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Jann Horn , Pedro Falcato , Hugh Dickins , Oscar Salvador , Lance Yang References: <20250717115212.1825089-1-david@redhat.com> <20250717115212.1825089-6-david@redhat.com> <46c9a90c-46b8-4136-9890-b9b2b97ee1bb@lucifer.local> <7701f2e8-ae17-4367-b260-925d1d3cd4df@redhat.com> <9184274d-2ae8-4949-a864-79693308bf56@lucifer.local> 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/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: <9184274d-2ae8-4949-a864-79693308bf56@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 18.07.25 12:41, Lorenzo Stoakes wrote: > On Thu, Jul 17, 2025 at 10:31:28PM +0200, David Hildenbrand wrote: >> On 17.07.25 20:29, Lorenzo Stoakes wrote: >>> On Thu, Jul 17, 2025 at 01:52:08PM +0200, David Hildenbrand wrote: >>>> The huge zero folio is refcounted (+mapcounted -- is that a word?) >>>> differently than "normal" folios, similarly (but different) to the ordinary >>>> shared zeropage. >>> >>> Yeah, I sort of wonder if we shouldn't just _not_ do any of that with zero >>> pages? >> >> I wish we could get rid of the weird refcounting of the huge zero folio and >> get rid of the shrinker. But as long as the shrinker exists, I'm afraid that >> weird per-process refcounting must stay. > > Does this change of yours cause any issue with it? I mean now nothing can grab > this page using vm_normal_page_pmd(), so it won't be able to manipulate > refcounts. Please look again at vm_normal_page_pmd(): we have a manual huge_zero_pfn() check in there! There is no change in behavior. :) It's not obvious from the diff below. But huge zero folio was considered special before this change, just not marked accordingly. >> >>> >>>> >>>> For this reason, we special-case these pages in >>>> vm_normal_page*/vm_normal_folio*, and only allow selected callers to >>>> still use them (e.g., GUP can still take a reference on them). >>>> >>>> vm_normal_page_pmd() already filters out the huge zero folio. However, >>>> so far we are not marking it as special like we do with the ordinary >>>> shared zeropage. Let's mark it as special, so we can further refactor >>>> vm_normal_page_pmd() and vm_normal_page(). >>>> >>>> While at it, update the doc regarding the shared zero folios. >>> >>> Hmm I wonder how this will interact with the static PMD series at [0]? >> >> No, it shouldn't. > > I'm always nervous about these kinds of things :) > > I'm assuming the reference/map counting will still work properly with the static > page? Let me stress again: no change in behavior besides setting the special flag in this patch. Return value of vm_normal_page_pmd() is not changed. >>> >>> Also, that series was (though I reviewed against it) moving stuff that >>> references the huge zero folio out of there, but also generally allows >>> access and mapping of this folio via largest_zero_folio() so not only via >>> insert_pmd(). >>> >>> So we're going to end up with mappings of this that are not marked special >>> that are potentially going to have refcount/mapcount manipulation that >>> contradict what you're doing here perhaps? >> >> I don't think so. It's just like having the existing static (small) shared >> zeropage where the same rules about refcounting+mapcounting apply. > > It feels like all this is a mess... am I missing something that would make it > all make sense? Let me clarify: The small zeropage is never refcounted+mapcounted when mapped into page tables. The huge zero folio is never refcounted+mapcounted when mapped into page tables EXCEPT it is refcounted in a weird different when first mapped into a process. The whole reason is the shrinker. I don't like it, but there was a reason it was added. Maybe that reason no longer exists, but that's nothing that this patch series is concerned with, really. :) -- Cheers, David / dhildenb