Re: [PATCH v2] dm-verity: support block number limits for different ioprio classes

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Mar 27, 2025 at 06:12:22PM +0100, Mikulas Patocka wrote:
> 
> 
> 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.

Interesting.  Unfortunately I think the workqueue makes it way worse.

In principle there is no need for it to block for anything.  If it would have to
block, it just should just continue on.

- Eric