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

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

On Mon, 16 Dec 2013 12:56:13 +0000 David Vrabel <david.vrabel@citrix.com> wrote:

> On 14/12/13 08:32, Andrew Morton wrote:
> > 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 :(
> 
> All the other callers do so from non-atomic context.  ghes.c is the only
> broken caller.
> 
> Shall I resend or are you happy to take the patch off the first email in
> this thread?

Well first we should attempt to wake up the ghes maintainers and tell
them we're about to break their stuff. (Which I believe is already broken).

The fix won't be easy - presumably ghes will need to punt all its IRQ-
and NMI_context operations up into kernel thread context.