On 9/27/22 19:14, Yang Shi wrote: > IIRC, the writeback may not call generic_writepages. On my ext4 > filesystem, the writeback call stack looks like: > > @[ > ext4_writepages+1 > do_writepages+191 > __writeback_single_inode+65 > writeback_sb_inodes+477 > __writeback_inodes_wb+76 > wb_writeback+457 > wb_workfn+680 > process_one_work+485 > worker_thread+80 > kthread+231 > ret_from_fork+34 > ]: 2 > Sure, that's fine for ext4, in that particular case, but a) not all filesystems have .writepages hooked up. That's why do_writepages() checks for .writepages(), and falls back to .writepage(): if (mapping->a_ops->writepages) ret = mapping->a_ops->writepages(mapping, wbc); else ret = generic_writepages(mapping, wbc); , and b) there are also a lot of places and ways to invoke writebacks. There are periodic writebacks, and there are data integrity (WB_SYNC_ALL) writebacks, and other places where a page needs to be cleaned--so, a lot of call sites. Some of which will land on a .writepage() sometimes, even now. For just one example, I see migrate.c's writeout() function directly calling writepage(): rc = mapping->a_ops->writepage(&folio->page, &wbc); thanks, -- John Hubbard NVIDIA
