Re: [BISECTED] Linux 3.12.7 introduces page map handling regression

From: Rik van Riel
Date: Wed Jan 22 2014 - 13:08:04 EST

On 01/21/2014 09:47 PM, Linus Torvalds wrote:
On Tue, Jan 21, 2014 at 5:49 PM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

Odds are this also shows up in 3.13, right?

Probably. I don't have a Xen PV setup to test with (and very little
interest in setting one up).. And I have a suspicion that it might not
be so much about Xen PV, as perhaps about the kind of hardware.

I suspect the issue has something to do with the magic _PAGE_NUMA
tie-in with _PAGE_PRESENT. And then mprotect(PROT_NONE) ends up
removing the _PAGE_PRESENT bit, and now the crazy numa code is

The whole _PAGE_NUMA thing is a f*cking horrible hack, and shares the
bit with _PAGE_PROTNONE, which is why it then has that tie-in to

The numa balancing code should clear _PAGE_PRESENT and

The difference between a numa pte and a protnone pte is
the VMA permissions.

When the VMA is protnone, do_page_fault will kill the
app with a segfault. When the VMA has proper permissions,
handle_pte_fault will call do_numa_page, and numa-y things
are done.

Adding Andrea to the Cc, because he's the author of that horridness.
Putting Steven's test-case here as an attachement for Andrea, maybe
that makes him go "Ahh, yes, silly case".

Also added Kirill, because he was involved the last _PAGE_NUMA debacle.

Andrea, you can find the thread on lkml, but it boils down to commit
1667918b6483 (backported to 3.12.7 as 3d792d616ba4) breaking the
attached test-case (but apparently only under Xen PV). There it
apparently causes a "BUG: Bad page map .." error.

And I suspect this is another of those "this bug is only visible on
real numa machines, because _PAGE_NUMA isn't actually ever set
otherwise". That has pretty much guaranteed that it gets basically
zero testing, which is not a great idea when coupled with that subtle
sharing of the _PAGE_PROTNONE bit..

It may be that the whole "Xen PV" thing is a red herring, and that
Steven only sees it on that one machine because the one he runs as a
PV guest under is a real NUMA machine, and all the other machines he
has tried it on haven't been numa. So it *may* be that that "only
under Xen PV" is a red herring. But that's just a possible guess.

Christ, how I hate that _PAGE_NUMA bit. Andrea: the fact that it gets
no testing on any normal machines is a major problem. If it was simple
and straightforward and the code was "obviously correct", it wouldn't
be such a problem, but the _PAGE_NUMA code definitely does not fall
under that "simple and obviously correct" heading.

Guys, any ideas?


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at