On Thu, 27 Mar 2025, Eric Biggers wrote: > > Hi, Eric > > > > It seems that verity_prefetch_io doesn't work efficiently. > > dm_bufio_prefetch_with_ioprio > > __dm_bufio_prefetch > > blk_start_plug > > for loop > > __bufio_new > > submit_io > > cond_resched > > blk_finish_plug > > > > If more than one hash blocks need to be prefetched, cond_resched will > > be called in each loop and blk_finish_plug will be called at the end. > > > > The requests for hash blocks may have not been dispatched when the > > requests for data blocks have been completed. > > Well, it's always possible for the prefetch I/O to be too slow, but the way it > works does seem to be unnecessarily bad. The prefetch work runs on a kworker, > which is going to slow things down. The way it should work is that verity_map() > runs the prefetch work and starts, but doesn't wait for, any I/O that is needed. > (And of course, most of the time no I/O will actually be needed.) > > - Eric dm-verity used to prefetch from the verity_map function, but it caused a deadlock - 3b6b7813b198b578aa7e04e4047ddb8225c37b7f - so, it was moved to a workqueue. Mikulas
