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(p200300d82f38ca00ca3a83da653e0234.dip0.t-ipconnect.de. [2003:d8:2f38:ca00:ca3a:83da:653e:234]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45617f18d99sm44212725e9.8.2025.07.14.08.41.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Jul 2025 08:41:46 -0700 (PDT) Message-ID: <184d7f4b-6ffd-4047-b245-aec32ebda9d6@redhat.com> Date: Mon, 14 Jul 2025 17:41:45 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c To: Pedro Falcato Cc: Lorenzo Stoakes , Andrew Morton , "Liam R . Howlett" , Vlastimil Babka , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jeff Xu References: <5d932ec1f9d0ea115aac65067e4cb8241a06e791.1752497324.git.lorenzo.stoakes@oracle.com> <5e21df9f-7f75-412b-a173-fe6da49952e5@redhat.com> <0925c64b-c721-4dc5-913a-c43a94dc64a3@redhat.com> From: David Hildenbrand Content-Language: en-US Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 14.07.25 17:31, Pedro Falcato wrote: > On Mon, Jul 14, 2025 at 05:03:03PM +0200, David Hildenbrand wrote: >> [...] >> >> But now I wonder, why is it okay to discard anon pages in a MAP_PRIVATE file >> mapping? > > IIRC this was originally suggested by Linus, on one of the versions introducing > mseal. But the gist is that discarding pages is okay if you could already zero > them manually, using e.g memset. Hence the writeability checks. What you can do is a) mmap(PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, ...) b) modify content (write, whatever) c) mprotect(PROT_READ) d) mseal() But then still do madvise(MADV_DONTNEED) to discard. There is no writability anymore. (Just a note that, with hugetlb, it is fairly common to mmap(MAP_PRIVATE) empty files and only work with anonymous pages.) -- Cheers, David / dhildenb