Messages in this thread | | | Date | Wed, 22 Jan 2014 13:07:44 -0500 | From | Rik van Riel <> | Subject | Re: [BISECTED] Linux 3.12.7 introduces page map handling regression |
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On 01/21/2014 09:47 PM, Linus Torvalds wrote: > On Tue, Jan 21, 2014 at 5:49 PM, Greg Kroah-Hartman > <gregkh@linuxfoundation.org> 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 > confused. > > 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 > _PAGE_PRESENT.
The numa balancing code should clear _PAGE_PRESENT and set _PAGE_NUMA / _PAGE_PROTNONE.
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? > > Linus >
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