On Wed, Mar 15, 2017 at 04:51:04PM -0500, Goldwyn Rodrigues wrote: > From: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx> > > A new flag BIO_NOWAIT is introduced to identify bio's > orignating from iocb with IOCB_NOWAIT. This flag indicates > to return immediately if a request cannot be made instead > of retrying. So this makes a congested block device run the bio IO completion callback with an -EAGAIN error present? Are all the filesystem direct IO submission and completion routines OK with that? i.e. does such a congestion case cause filesystems to temporarily expose stale data to unprivileged users when the IO is requeued in this way? e.g. filesystem does allocation without blocking, submits bio, device is congested, runs IO completion with error, so nothing written to allocated blocks, write gets queued, so other read comes in while the write is queued, reads data from uninitialised blocks that were allocated during the write.... Seems kinda problematic to me to have a undocumented design constraint (i.e a landmine) where we submit the AIO only to have it error out and then expect the filesystem to do something special and different /without blocking/ on EAGAIN. Why isn't the congestion check at a higher layer like we do for page cache readahead? i.e. using the bdi*congested() API at the time we are doing all the other filesystem blocking checks. -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
