Re: ubc_uiomove returns EINVAL

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Fri, 24 Oct 2014 09:49:03 -0700
    From:        Chuck Silvers <chuq%chuq.com@localhost>
    Message-ID:  <20141024164903.GA29200%spathi.chuq.com@localhost>

  | which is probably coming from
  | VOP_GETPAGES() of the vnode you're trying to access with ubc_uiomove().

This is the best clue on where to look next yet, thanks ... I assume
that must be from uvn_get() (in uvm_vnode.c) and is almost certainly
from

        error = uobj->pgops->pgo_get(uobj, uoff, &pg, &gotpages,
            0, flt->access_type & MASK(ufi->entry), ufi->entry->advice,
            PGO_SYNCIO);

in uvm_fault_lower_io() (uvm_fault.c) - that looks to be the only place
where VOP_GETPAGES() can get called from uvm_fault and actually return
an error (there's another place where the error, if any, looks to be ignored).

Does that sound reasonable, or am I missing something?

I see that this should be able to be tracked using the UVMHIST (aka KERNHIST)
stuff, but assuming I build a kernel with that enabled, how do I get access
to the data it collects?   I see it available via DDB, but is that the only
way, or is there some userland tool that can extract it?

This is, of course, just to verify that's the source of the error .. will
still need to look inside VOP_GETPAGES() (in the code it runs) to figure
out why it wants to return EINVAL, but one step at a time.

kre