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(p200300d82f38ca00ca3a83da653e0234.dip0.t-ipconnect.de. [2003:d8:2f38:ca00:ca3a:83da:653e:234]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b5e8bd1890sm12680696f8f.3.2025.07.14.08.52.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Jul 2025 08:52:57 -0700 (PDT) Message-ID: <60b0b01b-d397-4e47-8e85-295241ea3230@redhat.com> Date: Mon, 14 Jul 2025 17:52:56 +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: Lorenzo Stoakes Cc: Pedro Falcato , 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> <184d7f4b-6ffd-4047-b245-aec32ebda9d6@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 17:45, Lorenzo Stoakes wrote: > On Mon, Jul 14, 2025 at 05:41:45PM +0200, David Hildenbrand wrote: >> 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. > > Well, you can mprotect() writable it again :) Isn't that what sealing ... prohibits? -- Cheers, David / dhildenb