Re: [Xen-devel] PAE PV guest kernel regression

[Andrew Jones <drjones@xxxxxxxxxx>]

----- Original Message -----
> >>> Andrew Jones <drjones@xxxxxxxxxx> 06/07/12 12:47 PM >>>
> >Ah, I didn't check to see what Fedora pulled in on top of 3.4. Had I
> >done that I would have immediately suspected a different patch
> >instead
> >(mm-pmd_read_atomic-fix-32bit-PAE-pmd-walk-vs-pmd_populate-SMP-race-condition.patch,
> >upstream commit 26c191788f18). We've already encountered one problem
> >with this patch for RHEL6 and fixed it. The patch F17 has, however,
> >is
> >already the "fixed" version. Now the difference between RHEL6 and
> >F17
> >though is that F17 has CONFIG_TRANSPARENT_HUGEPAGE=y for 32b guests,
> >but RHEL6 does not. So now with this patch F17 is calling
> >atomic64_read() from pmd_none_or_trans_huge_or_clear_bad().
> 
> By itself the use of atomic64_read() on page table entries shouldn't
> be
> a problem though, particularly not if what might get written is a not
> present entry (while the way atoimc64_read_cx8() works would also
> allow for values with the low bit set to be pseudo-written, but
> neither
> did that appear to be the case in at least one of the two cases where
> register dumps were provided in your bugzilla, nor should an attempt
> to write back a value that was already there cause any problem, even
> if the low bit was set).
> 
> In any case, the hypervisor log would be interesting to see. Plus -
> is
> this perhaps a rather old hypervisor running there (i.e. potentially
> lacking some fix)?

I just reproduced this on my own test box, which is running an HV
based on older code (it's RHEL 5.8). Even with
'loglvl=all guest_loglvl=all' I didn't get anything logged in
'xm dmesg'.

Drew

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