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Thu, 5 Nov 2026 12:19:15 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 3z2mNUJ07GqdHgAAD6G6ig (envelope-from ); Thu, 05 Nov 2026 12:19:15 +0000 Date: Thu, 5 Nov 2026 12:19:13 +0000 From: Pedro Falcato To: "Jose Fernandez (Anthropic)" Cc: "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , David Hildenbrand , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Krystian Kaniewski , stable@vger.kernel.org Subject: Re: [PATCH v2] mm/filemap: stop fault-around when the walk steps back Message-ID: References: <20261031-jf-mm-filemap-map-pages-lower-bound-v2-1-21029744e589@linux.dev> <20261104-jf-filemap-v2-pedro-reply-r2-74c87c6fbcf@linux.dev> 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=us-ascii Content-Disposition: inline In-Reply-To: <20261104-jf-filemap-v2-pedro-reply-r2-74c87c6fbcf@linux.dev> 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)[]; FREEMAIL_CC(0.00)[infradead.org,suse.cz,linux-foundation.org,kernel.org,vger.kernel.org,kvack.org,gmail.com]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_ALL(0.00)[]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCVD_TLS_ALL(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCPT_COUNT_SEVEN(0.00)[10]; MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spam-Score: -13.51 X-Spam-Level: X-Rspamd-Queue-Id: E01831F458 On Wed, Nov 04, 2026 at 11:12:41PM +0000, Jose Fernandez (Anthropic) wrote: > On Tue, Nov 03, 2026 at 12:12:58PM +0000, Pedro Falcato wrote: > > I'm not sure I understand. xas_find() was fixed. Is there any other case where > > this loop can step backwards? > > > > If not, I don't think this should be Cc: stable, and should probably be a WARN_ON(). > > As it's a hardening patch and not an actual bugfix. > > Yes, there is at least one more case. It triggers when a read fault > starts a lookup inside a large folio while a hole punch removes that > folio and a write puts part of it back. A stress test I wrote for the Thank you. So, if I'm not mistaken, this is the race: First, you map/see this: [folio pgoff 4, length 4] Then, at some point in time, someone punches the hole: [no folios here] Then, at some point in time, someone faults another folio in the range: [folio pgoff 0, length 8] so you step backwards. Is this correct? Or am I misunderstanding this? If so, how can this happen while we hold the PTE lock? That itself should block truncation. Can you share the reproducer? -- Pedro