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
