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Tue, 27 Oct 2026 13:05:05 +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 gzdcGYGh4GpsMgAAD6G6ig (envelope-from ); Tue, 27 Oct 2026 13:05:05 +0000 Date: Tue, 27 Oct 2026 13:05:03 +0000 From: Pedro Falcato To: "David Hildenbrand (Arm)" Cc: Lance Yang , akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, kas@kernel.org, jack@suse.cz, djwong@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, hch@lst.de, ngocthang2710.1999@gmail.com, syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com, stable@vger.kernel.org Subject: Re: [PATCH v4] khugepaged: fix readahead lock inversion in collapse_file() Message-ID: References: <20261026152013.10511-1-lance.yang@linux.dev> <321017a4-58cb-4178-8ff2-055a9bbb28b6@kernel.org> 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: <321017a4-58cb-4178-8ff2-055a9bbb28b6@kernel.org> X-Spamd-Result: default: False [-12.01 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; 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)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCPT_COUNT_TWELVE(0.00)[20]; MIME_TRACE(0.00)[0:+]; FREEMAIL_CC(0.00)[linux.dev,linux-foundation.org,nvidia.com,linux.alibaba.com,infradead.org,arm.com,kernel.org,suse.cz,kvack.org,vger.kernel.org,lst.de,gmail.com,syzkaller.appspotmail.com]; DKIM_TRACE(0.00)[suse.de:+]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCVD_TLS_ALL(0.00)[]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received]; TAGGED_RCPT(0.00)[16bf7cd0ebeb1de93aa5]; 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-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spam-Score: -12.01 X-Spam-Level: X-Rspamd-Queue-Id: 1506D21F1B On Tue, Oct 27, 2026 at 12:06:05PM +0100, David Hildenbrand (Arm) wrote: > On 10/26/26 16:20, Lance Yang wrote: > > collapse_file() can call page_cache_sync_readahead() while still holding > > source folios locked. If truncate holds mapping->invalidate_lock > > exclusively and waits for one of those folios, readahead waits for > > invalidate_lock and neither side can make progress. > > > > We need to release those folio locks before starting readahead. On a page > > cache miss, roll back first, populate the collapse range, and retry once. > > Doing readahead after rollback avoids an extra pass when all folios are > > already present. If another miss occurs on retry, give up rather than keep > > retrying while truncate or reclaim removes folios. > > Makes sense. So we simply try readahead on all folios. > > Any reason we don't simply add a second loop after the first loop where we don't > take the folio locks and do the swapin/readahead dance, then a second loop where > we do all the other stuff? I'm not sure I understand. You want to nest two loops? Or do you want a loop upfront? I don't think we want a loop upfont. That's slow and involves starting/waiting for IO. Fallback logic makes more sense IMO. FWIW, I plan on rebasing my refactoring patches for 7.5, and hopefully knock some sense into this code. That will hopefully make this cleaner. -- Pedro