svn commit: r340187 - head/sys/geom

[Rodney W. Grimes]

> On Tue, 2018-11-06 at 16:17 -0800, Rodney W. Grimes wrote:
> > > 
> > > Rodney, this was actually my original intention, however then I noticed in
> > > the GEOM code there is at least one case when BIO_FLUSH request is being
> > > generated internally with bio_offset == mediasize and bio_lenth == 0, so I
> > > thought there might be some need to allow such requests through. But I'd
> > > happily go with the stricter rule if it does no harm. I simply don't know
> > > enough about the intended use and the logic behind zero-length transfers to
> > > make that call.
> > I am not sure enough on if mediasize is 0 based or not,
> > if it is then the error case should be fixed, and the
> > code you show below should also be fixed as it is
> > technically making a request beyond the end of device.
> > 
> > I am also murky on why we are even doing a 0 size
> > operation and end of device, is that to validate
> > we can access all the media???If so then this wrong
> > code and wrong error return should be fixed as it
> > is off by 1.
> > 
> > > 
> > > 
> > > -Max
> > > 
> > > int
> > > g_io_flush(struct g_consumer *cp)
> > > {
> > > ...
> > > ????????bp = g_alloc_bio();
> > > ????????bp->bio_cmd = BIO_FLUSH;
> > > ...
> > > ????????bp->bio_offset = cp->provider->mediasize;
> > The above should have a - 1 on it.
> > 
> 
> Unless offset > mediasize is specifically a signal to downstream code
> in some way about how the flush is to be performed.

Could very well be, should be documented some place though.

> Nearly identical code to create a BIO_FLUSH bio appears in ufs softdeps
> and in zfs. Before starting to arbitrarily change code that has worked
> since 2006, it might be a good idea to track down why these values are
> set the way they are. Unfortunately, there is no clue in the commit
> logs, but maybe the author (pjd@, cc'd) can englighten us.

I agree with that take on the situation, and it is why I asked
for a revert and investigation, rather than trying to solve
why we suddenly fail some regression tests.

-- 
Rod Grimes                                                 rgrimes at freebsd.org