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[2003:cb:c721:b900:4f34:b2b7:739d:a650]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429ded299a9sm81171405e9.18.2024.08.16.11.08.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Aug 2024 11:08:40 -0700 (PDT) Message-ID: <94c5d735-821c-40ba-ae85-1881c6f4445d@redhat.com> Date: Fri, 16 Aug 2024 20:08:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages To: Ackerley Tng , Fuad Tabba Cc: Elliot Berman , Andrew Morton , Paolo Bonzini , Sean Christopherson , Patrick Roy , qperret@google.com, linux-coco@lists.linux.dev, linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org References: <20240805-guest-memfd-lib-v1-0-e5a29a4ff5d7@quicinc.com> <20240805-guest-memfd-lib-v1-4-e5a29a4ff5d7@quicinc.com> <4cdd93ba-9019-4c12-a0e6-07b430980278@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: rspam12 X-Rspamd-Queue-Id: 3CEF880005 X-Stat-Signature: uto58zy7jrzk19ki87skhwdustdqn3d9 X-Rspam-User: X-HE-Tag: 1723831724-345143 X-HE-Meta: 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 IFcYm+7v XxJUfW3hDgwHMMk8JXBSmNnbjLpn5o+SagTY1/Oy1FY51dQTE/2ilnyADAjKgSlnnxTwetDwZn1AqrDnGjHC0SXn8JRgCgttbjfa/g4JZZW/rA0vP186lCRn6YpD9uqHlfYUb4VCKRMC+HXFwVj8RxlIETxKGJFl17+QRGVpWzOpzUoJM5/zgmjI0iFtAAO2p42GNyYsF9FpqYOsrREJfFOhdJAMXyaW+G4JYJfj6K2qC/Re2+Rw6v6NkIilX9WjD8NNypY6YXc3STXkAQM7JJvE6J3JMxmUW56DfR7SDgc+srQbLuX96e06SQsAJzra77QiET4+9aelr772zLm000YTQKxdoeZjiRL2EOCIYFIkGdg3mv1Z+MJP5J9qk6qig+Z1DWc82zY06cg0b1YQNqT0nrQ== 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 16.08.24 19:45, Ackerley Tng wrote: > > David Hildenbrand writes: > >> On 15.08.24 09:24, Fuad Tabba wrote: >>> Hi David, >> >> Hi! >> >>> >>> On Tue, 6 Aug 2024 at 14:51, David Hildenbrand wrote: >>>> >>>>> >>>>> - if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) { >>>>> + if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) { >>>>> r = guest_memfd_folio_private(folio); >>>>> if (r) >>>>> goto out_err; >>>>> @@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags >>>>> } >>>>> EXPORT_SYMBOL_GPL(guest_memfd_grab_folio); >>>>> >>>>> +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio) >>>>> +{ >>>>> + unsigned long gmem_flags = (unsigned long)file->private_data; >>>>> + unsigned long i; >>>>> + int r; >>>>> + >>>>> + unmap_mapping_folio(folio); >>>>> + >>>>> + /** >>>>> + * We can't use the refcount. It might be elevated due to >>>>> + * guest/vcpu trying to access same folio as another vcpu >>>>> + * or because userspace is trying to access folio for same reason >>>> >>>> As discussed, that's insufficient. We really have to drive the refcount >>>> to 1 -- the single reference we expect. >>>> >>>> What is the exact problem you are running into here? Who can just grab a >>>> reference and maybe do nasty things with it? >>> >>> I was wondering, why do we need to check the refcount? Isn't it enough >>> to check for page_mapped() || page_maybe_dma_pinned(), while holding >>> the folio lock? > > Thank you Fuad for asking! > >> >> (folio_mapped() + folio_maybe_dma_pinned()) >> >> Not everything goes trough FOLL_PIN. vmsplice() is an example, or just >> some very simple read/write through /proc/pid/mem. Further, some >> O_DIRECT implementations still don't use FOLL_PIN. >> >> So if you see an additional folio reference, as soon as you mapped that >> thing to user space, you have to assume that it could be someone >> reading/writing that memory in possibly sane context. (vmsplice() should >> be using FOLL_PIN|FOLL_LONGTERM, but that's a longer discussion) >> > > Thanks David for the clarification, this example is very helpful! > > IIUC folio_lock() isn't a prerequisite for taking a refcount on the > folio. Right, to do folio_lock() you only have to guarantee that the folio cannot get freed concurrently. So you piggyback on another reference (you hold indirectly). > > Even if we are able to figure out a "safe" refcount, and check that the > current refcount == "safe" refcount before removing from direct map, > what's stopping some other part of the kernel from taking a refcount > just after the check happens and causing trouble with the folio's > removal from direct map? Once the page was unmapped from user space, and there were no additional references (e.g., GUP, whatever), any new references can only be (should, unless BUG :) ) temporary speculative references that should not try accessing page content, and that should back off if the folio is not deemed interesting or cannot be locked. (e.g., page migration/compaction/offlining). Of course, there are some corner cases (kgdb, hibernation, /proc/kcore), but most of these can be dealt with in one way or the other (make these back off and not read/write page content, similar to how we handled it for secretmem). These (kgdb, /proc/kcore) might not even take a folio reference, they just "access stuff" and we only have to teach them to "not access that". > >> (noting that also folio_maybe_dma_pinned() can have false positives in >> some cases due to speculative references or *many* references). > > Are false positives (speculative references) okay since it's better to > be safe than remove from direct map prematurely? folio_maybe_dma_pinned() is primarily used in fork context. Copying more (if the folio maybe pinned and, therefore, must not get COW-shared with other processes and must instead create a private page copy) is the "better safe than sorry". So false positives (that happen rarely) are tolerable. Regading the directmap, it would -- just like with additional references -- detect that the page cannot currently be removed from the direct map. It's similarly "better safe than sorry", but here means that we likely must retry if we cannot easily fallback to something else like for the fork+COW case. -- Cheers, David / dhildenb