CVE-2026-98538: writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes

In the Linux kernel, the following vulnerability has been resolved:

writeback: bound cleanupofflinecgwb() rescans by rotating scanned inodes

cleanupofflinecgwb() prepares at most WBMAXINODESPERISW inodes per call and is called again until the dying wb is drained, but every call walks wb->battached and then wb->bdirtytime from the same end. Inodes already prepared (they stay on the list with IWBSWITCH set until the switch worker runs) and inodes that cannot be switched (IFREEING, IWILLFREE, !SBACTIVE, DAX, already on the target wb) stay where they are, so each pass rescans a growing run of them under wb->listlock and a full drain is quadratic in the number of inodes on the list. With ~17M inodes attached to one dying cgwb we saw this end in soft lockups, with CPUs reported stuck for 21-48s.

Walk both lists from the oldest end and move every scanned inode to the newest end, so the next pass starts where the previous one stopped and the drain becomes linear. battached is unordered, so nobody sees the reorder there. bdirtytime is ordered by dirtiedwhen, but the oldest unscanned inode stays at the end moveexpiredinodes() picks from, sync takes the whole list regardless of order, and prepared inodes leave the list as soon as the switch work runs and get a new dirtiedtimewhen on the new wb anyway, so the only inodes left out of order are the ones that can never switch (DAX), and only on the dying wb.