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

David Vrabel
2013-12-16T12:56:13Z

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?

David