On Tue, Jan 05, 2021 at 10:54:19AM -0300, Mauricio Faria de Oliveira wrote: > There's an I/O error on fsync() in a detached loop device > if it has been previously attached. > > The issue is write cache is enabled in the attach path in > loop_configure() but it isn't disabled in the detach path; > thus it remains enabled in the block device regardless of > whether it is attached or not. > > Now fsync() can get an I/O request that will just be failed > later in loop_queue_rq() as device's state is not 'Lo_bound'. > > So, disable write cache in the detach path. > > Test-case: > > # DEV=/dev/loop7 > > # IMG=/tmp/image > # truncate --size 1M $IMG > > # losetup $DEV $IMG > # losetup -d $DEV > > Before: > > # strace -e fsync parted -s $DEV print 2>&1 | grep fsync > fsync(3) = -1 EIO (Input/output error) > Warning: Error fsyncing/closing /dev/loop7: Input/output error > [ 982.529929] blk_update_request: I/O error, dev loop7, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0 > > After: > > # strace -e fsync parted -s $DEV print 2>&1 | grep fsync > fsync(3) = 0 But IO on detached loop should have been failed, right? The magic is that submit_bio_checks() filters FLUSH request for queues which doesn't support writeback cache, and always fake a normal completion. I understand that the issue is that user becomes confused with this observation because no such failure if they run 'parted -s /dev/loop0 print' on one detached loop disk if it is never attached. Thanks, Ming
