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

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Message-ID: <52AEF86D.808@citrix.com>
Date:	Mon, 16 Dec 2013 12:56:13 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 14/12/13 08:32, Andrew Morton wrote:
> 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 :(

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?

David

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