[Lguest] [PATCH] fix lguest rmmod "bad pgd"

[Rusty Russell]

On Thursday 03 January 2008 00:27:10 pageexec at freemail.hu wrote:
> (sorry for breaking the thread but i didn't get the original headers)
>
> > After 17d57a9206b4de6ad082ac9f2d2346985abbd2aa
> > (x86: fix x86-32 early fixmap initialization.) removing lg.ko caused a
> > printk from vunmap:
> >
> >  mm/memory.c:115: bad pgd 004b3027.
> >
> > On the second use after module load, the kernel crashes.
> >
> > This fixes the immediate problem (accessed and dirty bits not set as
> > expected in pmd_none_or_clear_bad).  I can't see why this would cause a
> > crash, but I haven't been able to reproduce it once this is applied.
>
> it's the 'clear_bad' part that zero's out the pmd and hence destroys
> all the fixmap mappings in that 4 MB range leading to page faults at
> probably the most unexpected times.

Thanks, I had misread the code as clearing it normally anyway.  This is a 
nasty bug because we'd hit it if we ever filled vmalloc space, but now Linus 
has applied that patch we should all be merry...

Cheers,
Rusty.