In lib/pagewalk.c, I've been using the various forms of

{pgd,pud,pmd}_none_or_clear_bad while walking page tables as that

seemed the canonical way to do things. Lately (eg with -rc7-mm1),

these have been triggering messages like "bad pgd 0x01e3" and causing

nasty double faults. It appears this is actually triggered at the pmd

level (mm/memory.c:116), though it appears to produce the wrong

message.
Has something changed here? I'm pretty sure this used to work! Is this

not a kosher thing to do? Does it make any sense I'd repeatedly run

into a bad pmd in the middle of bash's page table right after boot?

The simple _none variant seems to work, but I worry that it's papering

over a real problem.
--

Mathematics is the supreme nostalgia of our time.

-

No, I think that should be the right thing to do for userspace pages.

You're not walking into a hugetlb area or a kernel mapping are you?

(the bad pgd: line could be important... 0x01e3 would be a linear kernel

mapping I think?).
-

I guess the "wrong message" is an artifact of pud/pmd folding;

but I get too confused by the different levels myself to want to
I should have spent more time reading Nick's reply and less time trying

to work it out for myself!  Yes, that's the conclusion I came to, for

some reason you're now going beyond the user vmas and walking into the

linear kernel mapping, which has _PAGE_GLOBAL and _PAGE_PSE bits set.
Hugh

-

Indeed, that's precisely what's happening. I'm walking one page past

the end of userspace. 
And the reason is I changed my walker from using for loops to do/while

loops at Nick's insistance, so start==end no longer gets noticed

immediately. This also explains why the bug doesn't manifest in

lguest: no PSE mappings.
Thanks, guys!
--

Mathematics is the supreme nostalgia of our time.

-