Re: vunmap() on large regions may trigger soft lockup warnings

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 12 Dec 2013 12:50:47 +0000 David Vrabel <david.vrabel@citrix.com> wrote:

> > each time.  But that would require difficult tuning of N.
> > 
> > I suppose we could just do
> > 
> > 	if (!in_interrupt())
> > 		cond_resched();
> > 
> > in vunmap_pmd_range(), but that's pretty specific to ghes.c and doesn't
> > permit unmap-inside-spinlock.
> > 
> > So I can't immediately think of a suitable fix apart from adding a new
> > unmap_kernel_range_atomic().  Then add a `bool atomic' arg to
> > vunmap_page_range() and pass that all the way down.
> 
> That would work for the unmap, but looking at the GHES driver some more
> and it looks like it's call to ioremap_page_range() is already unsafe --
> it may need to allocate a new PTE page with a non-atomic alloc in
> pte_alloc_one_kernel().
> 
> Perhaps what's needed here is a pair of ioremap_page_atomic() and
> iounmap_page_atomic() calls?  With some prep function to sure the PTE
> pages (etc.) are preallocated.

Is ghes.c the only problem source here?  If so then a suitable solution
would be to declare that driver hopelessly busted and proceed as if it
didn't exist :(

Just from a quick look, the thing is doing ioremap() from NMI context! 
ioremap has to do a bunch of memory allocations, takes spinlocks etc.