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Thu, 11 Jul 2024 12:07:59 -0700 (PDT) Received: from [10.35.209.243] ([208.115.86.72]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-70b4398002bsm6224007b3a.148.2024.07.11.12.07.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jul 2024 12:07:59 -0700 (PDT) Message-ID: <37da7835-0d76-463e-b074-455e405b138b@redhat.com> Date: Thu, 11 Jul 2024 21:07:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings To: Linus Torvalds , "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, patches@lists.linux.dev, tglx@linutronix.de, linux-crypto@vger.kernel.org, linux-api@vger.kernel.org, x86@kernel.org, Greg Kroah-Hartman , Adhemerval Zanella Netto , Carlos O'Donell , Florian Weimer , Arnd Bergmann , Jann Horn , Christian Brauner , David Hildenbrand , linux-mm@kvack.org References: <20240709130513.98102-1-Jason@zx2c4.com> <20240709130513.98102-2-Jason@zx2c4.com> <378f23cb-362e-413a-b221-09a5352e79f2@redhat.com> <9b400450-46bc-41c7-9e89-825993851101@redhat.com> 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: 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-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 9B33140030 X-Stat-Signature: fnx3k6o7jjhrxpi63c18x76qn8ugjcq6 X-Rspam-User: X-HE-Tag: 1720724884-450089 X-HE-Meta: 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 wHWAKC52 gL+5yXdKWujYn3XgUtZy2pGZnfsGoYIFy20bPP2lQUcSt064lsqVXjVGmcfMrdV7t5uq5hnEDJuX7ChzYsB5c5fkg0d5OxrjMq/zgYoqooqFKPO3Lm+So9XGT8HMR10liSrf0WTJ0QMxZmhmdgn7B7cO0fP4FGazUPtdeKJZuxLeVTUdpGbVUREU/qXksZL4dT67//y1rPcD7IoGhX/BCsRWyzjtjWylH+J6N3CTfVuu+XSruMac9WdwSj1cIuihRoLwM3tAmMT0azK4Fc8Z3fVHMAqUak/cx+Omr9K1tbALS8kklAKtGaA7xqGx5KiL1DhL7D4xt+4yvwBSmVhiEDxZo6H0wK1p6h9gCHBE2fMgDFjVVKfAe/W4I+/Ni+foYwlcEgVMCK4R9emP83Pp2WYnnMZeUdPgXfhRqKNXIQkXezzb67P7DwVWHQq9/eum/x6a3jNwORHa1bbFjYthUuw5xpNdR5PULrjwDJicsEroxBFtFMECgBgbAtw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11.07.24 19:57, Linus Torvalds wrote: > On Thu, 11 Jul 2024 at 10:09, Jason A. Donenfeld wrote: >> >> When I was working on this patchset this year with the syscall, this is >> similar somewhat to the initial approach I was taking with setting up a >> special mapping. It turned into kind of a mess and I couldn't get it >> working. There's a lot of functionality built around anonymous pages >> that would need to be duplicated (I think?). > > Yeah, I was kind of assuming that. You'd need to handle VM_DROPPABLE > in the fault path specially, the way we currently split up based on > vma_is_anonymous(), eg > > if (vma_is_anonymous(vmf->vma)) > return do_anonymous_page(vmf); > else > return do_fault(vmf); > > in do_pte_missing() etc. > > I don't actually think it would be too hard, but it's a more > "conceptual" change, and it's probably not worth it. > >> Alright, an hour later of fiddling, and it doesn't actually work (yet?) >> -- the selftest fails. A diff follows below. > > May I suggest a slightly different approach: do what we did for "pte_mkwrite()". > > It needed the vma too, for not too dissimilar reasons: special dirty > bit handling for the shadow stack. See > > bb3aadf7d446 ("x86/mm: Start actually marking _PAGE_SAVED_DIRTY") > b497e52ddb2a ("x86/mm: Teach pte_mkwrite() about stack memory") > > and now we have "pte_mkwrite_novma()" with the old semantics for the > legacy cases that didn't get converted - whether it's because the > architecture doesn't have the issue, or because it's a kernel pte. > > And the conversion was actually quite pain-free, because we have > > #ifndef pte_mkwrite > static inline pte_t pte_mkwrite(pte_t pte, struct vm_area_struct *vma) > { > return pte_mkwrite_novma(pte); > } > #endif > > so all any architecture that didn't want this needed to do was to > rename their pte_mkwrite() to pte_mkwrite_novma() and they were done. > In fact, that was done first as basically semantically no-op patches: > > 2f0584f3f4bd ("mm: Rename arch pte_mkwrite()'s to pte_mkwrite_novma()") > 6ecc21bb432d ("mm: Move pte/pmd_mkwrite() callers with no VMA to _novma()") > 161e393c0f63 ("mm: Make pte_mkwrite() take a VMA") > > which made this all very pain-free (and was largely a sed script, I think). > >> - !pte_dirty(pte) && !PageDirty(page)) >> + !pte_dirty(pte) && !PageDirty(page) && >> + !(vma->vm_flags & VM_DROPPABLE)) > > So instead of this kind of thing, we'd have > >> - !pte_dirty(pte) && !PageDirty(page)) >> + !pte_dirty(pte, vma) && !PageDirty(page) && > > and the advantage here is that you can't miss anybody by mistake. The > compiler will be very unhappy if you don't pass in the vma, and then > any places that would be converted to "pte_dirty_novma()" > > We don't actually have all that many users of pte_dirty(), so it > doesn't look too nasty. And if we make the pte_dirty() semantics > depend on the vma, I really think we should do it the same way we did > pte_mkwrite(). We also have these folio_mark_dirty() calls, for example in unpin_user_pages_dirty_lock(). Hm ... so preventing the folio from getting dirtied is likely shaky. I guess we need a way to just reliably identify these folios :/. -- Cheers, David / dhildenb