svn commit: r340187 - head/sys/geom

[Ian Lepore]

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.

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.

-- Ian