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(p200300cbc70dfb00d3ed5f441b2d12af.dip0.t-ipconnect.de. [2003:cb:c70d:fb00:d3ed:5f44:1b2d:12af]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4398148f4fcsm71828975e9.7.2025.02.18.09.00.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Feb 2025 09:00:38 -0800 (PST) Message-ID: Date: Tue, 18 Feb 2025 18:00:36 +0100 Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> <195a2b04-2968-4a51-9308-855da9fc12ed@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/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: <195a2b04-2968-4a51-9308-855da9fc12ed@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 18.02.25 17:49, Lorenzo Stoakes wrote: > On Tue, Feb 18, 2025 at 05:27:24PM +0100, David Hildenbrand wrote: >> 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. > > More docs noooo! :P I will update the man pages when this is more obviously > heading for landing in 6.15 accordingly. > > Current man page documentation on this is: > > 'If the region maps memory pages those mappings will be replaced as part of > the operation' > > I think something like: > > 'If the region maps pages those mappings will be replaced as part of the > operation. When guard regions are removed via MADV_GUARD_REMOVE, faulting > in the page will behave as if that region had MADV_DONTNEED applied to it, > that is anonymous ranges will be backed by newly allocated zeroed pages and > file-backed ranges will be backed by the underlying file pages.' > > Probably something less wordy than this... Yeah, but sounds good to me. > >> >>> >>>> 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 :) > > I am talking about both, I multitask ;) For !shmem, we should indeed not be messing with a sparse file structure. -- Cheers, David / dhildenb