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(p200300cbc70dfb00d3ed5f441b2d12af.dip0.t-ipconnect.de. [2003:cb:c70d:fb00:d3ed:5f44:1b2d:12af]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f258b439dsm15274522f8f.8.2025.02.18.08.27.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Feb 2025 08:27:26 -0800 (PST) Message-ID: Date: Tue, 18 Feb 2025 17:27:24 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] mm: allow guard regions in file-backed and read-only mappings To: Lorenzo Stoakes Cc: Andrew Morton , Suren Baghdasaryan , "Liam R . Howlett" , Matthew Wilcox , Vlastimil Babka , "Paul E . McKenney" , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shuah Khan , linux-kselftest@vger.kernel.org, linux-api@vger.kernel.org, John Hubbard , Juan Yescas , Kalesh Singh References: <6500a93f-aad1-4b21-a94e-feb493c344a3@redhat.com> <4d6d2942-10a1-46e8-93a6-7ce52b6af3ad@lucifer.local> <90a00957-02b5-440a-9168-de93c760fea7@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-MFC-PROC-ID: 5RxXD83qQSL8f1Dc3KNec7jPrm3X8izvHgmNKQK5gKo_1739896047 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 0B0DB160003 X-Stat-Signature: egbx3mtpbxfm1r839bs5b93syc4h1a6a X-HE-Tag: 1739896049-526876 X-HE-Meta: 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 YuOEH0Y/ 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 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 18.02.25 17:21, Lorenzo Stoakes wrote: > On Tue, Feb 18, 2025 at 05:17:20PM +0100, David Hildenbrand wrote: >> On 18.02.25 17:12, Lorenzo Stoakes wrote: >>> On Tue, Feb 18, 2025 at 05:01:16PM +0100, David Hildenbrand wrote: >>>> On 13.02.25 19:17, Lorenzo Stoakes wrote: >>>>> There is no reason to disallow guard regions in file-backed mappings - >>>>> readahead and fault-around both function correctly in the presence of PTE >>>>> markers, equally other operations relating to memory-mapped files function >>>>> correctly. >>>>> >>>>> Additionally, read-only mappings if introducing guard-regions, only >>>>> restrict the mapping further, which means there is no violation of any >>>>> access rights by permitting this to be so. >>>>> >>>>> Removing this restriction allows for read-only mapped files (such as >>>>> executable files) correctly which would otherwise not be permitted. >>>>> >>>>> Signed-off-by: Lorenzo Stoakes >>>>> --- >>>>> mm/madvise.c | 8 +------- >>>>> 1 file changed, 1 insertion(+), 7 deletions(-) >>>>> >>>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>>> index 6ecead476a80..e01e93e179a8 100644 >>>>> --- a/mm/madvise.c >>>>> +++ b/mm/madvise.c >>>>> @@ -1051,13 +1051,7 @@ static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked) >>>>> if (!allow_locked) >>>>> disallowed |= VM_LOCKED; >>>>> - if (!vma_is_anonymous(vma)) >>>>> - return false; >>>>> - >>>>> - if ((vma->vm_flags & (VM_MAYWRITE | disallowed)) != VM_MAYWRITE) >>>>> - return false; >>>>> - >>>>> - return true; >>>>> + return !(vma->vm_flags & disallowed); >>>>> } >>>>> static bool is_guard_pte_marker(pte_t ptent) >>>> >>>> Acked-by: David Hildenbrand >>> >>> Thanks! >>> >>>> >>>> I assume these markers cannot completely prevent us from allocating >>>> pages/folios for these underlying file/pageache ranges of these markers in >>>> case of shmem during page faults, right? >>> >>> If the markers are in place, then page faulting will result in a >>> segfault. If we faulted in a shmem page then installed markers (which would >>> zap the range), then the page cache will be populated, but obviously >>> subject to standard reclaim. >> >> Well, yes, (a) if there is swap and (b), if the noswap option was not >> specified for tmpfs. >> > > Right, yeah if you don't have it set up such that dropping a reference to the > folio doesn't drop the page altogether. > > I think this matches expectation though in that you'd get the same results from > an MADV_DONTNEED followed by faulting the page again. It might make sense to document that: installing a guard behaves just like MADV_DONTNEED; in case of a file, that means that the pagecache is left untouched. > >> Okay, so installing a guard entry might require punshing a hole to get rid >> of any already-existing memory. But readahead (below) might mess it up. > > Only if you are so concerned about avoiding the page cache being populated there > that you want to do this :) > > Readahead I think will not readahead into a holepunched region as the hole > punching extends to the fs layer _I believe_ I have not checked the code for > this, but I believe it actually changes the underlying file too right to say > 'this part of the file is empty'? Well, we are talking about shmem here ... not your ordinary fs backed by an actual file :) -- Cheers, David / dhildenb