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

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Date:	Sat, 14 Dec 2013 00:32:08 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	<linux-acpi@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xenproject.org>,
	Dietmar Hahn <dietmar.hahn@...fujitsu.com>
Subject: Re: vunmap() on large regions may trigger soft lockup warnings

On Thu, 12 Dec 2013 12:50:47 +0000 David Vrabel <david.vrabel@...rix.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.

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