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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/khugepaged: reject shmem collapse ranges beyond i_size Message-ID: References: <20261112142335.2749597-2-Jeremy.Jean@oss.cyber.gouv.fr> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20261112142335.2749597-2-Jeremy.Jean@oss.cyber.gouv.fr> X-Rspamd-Queue-Id: 278691F7CE X-Spam-Level: X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spamd-Result: default: False [-13.51 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCPT_COUNT_TWELVE(0.00)[16]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -13.51 On Thu, Nov 12, 2026 at 02:23:35PM +0000, Jérémy Jean wrote: > MADV_COLLAPSE lets collapse_file() treat absent shmem pages as holes > filled with zeros. It does not check i_size, which allows one present > page to become a huge mapping that covers the whole collapse range, > including pages beyond EOF. This makes those pages writable. A later > file extension can expose their contents after F_SEAL_WRITE. > > Hold inode i_rwsem in read mode while collapsing shmem so i_size > cannot change. Reject a collapse if its page range extends past > i_size rounded up to PAGE_SIZE. Keep i_size_read() outside the > page-cache XArray lock because its 32-bit SMP reader is not IRQ > safe. I don't think this is an issue. shmem/tmpfs purposefully maps with large pages across i_size, and collapsing across i_size is a feature, not a bug. -- Pedro