Re: [Xen-devel] vunmap() on large regions may trigger soft lockup warnings

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 11/12/13 21:39, Andrew Morton wrote:
> On Wed, 11 Dec 2013 16:58:19 +0000 David Vrabel <david.vrabel@xxxxxxxxxx> 
> wrote:
> 
>> Andrew,
>>
>> Dietmar Hahn reported an issue where calling vunmap() on a large (50 GB)
>> region would trigger soft lockup warnings.
>>
>> The following patch would resolve this (by adding a cond_resched() call
>> to vunmap_pmd_range()). Almost calls of vunmap(), unmap_kernel_range()
>> are from process context (as far as I could tell) except for an ACPI
>> driver (drivers/acpi/apei/ghes.c) calls unmap_kernel_range_noflush()
>> from an interrupt and NMI contexts.
>>
>> Can you advise on a preferred solution?
>>
>> For example, an unmap_kernel_page() function (callable from atomic
>> context) could be provided since the GHES driver only maps/unmaps a
>> single page.
>>
>> 8<-------------------------
>> mm/vmalloc: avoid soft lockup warnings when vunmap()'ing large ranges
>>
>> From: David Vrabel <david.vrabel@xxxxxxxxxx>
>>
>> If vunmap() is used to unmap a large (e.g., 50 GB) region, it may take
>> sufficiently long that it triggers soft lockup warnings.
>>
>> Add a cond_resched() into vunmap_pmd_range() so the calling task may
>> be resheduled after unmapping each PMD entry.  This is how
>> zap_pmd_range() fixes the same problem for userspace mappings.
>>
>> ...
>>
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -75,6 +75,7 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long
>> addr, unsigned long end)
>>              if (pmd_none_or_clear_bad(pmd))
>>                      continue;
>>              vunmap_pte_range(pmd, addr, next);
>> +            cond_resched();
>>      } while (pmd++, addr = next, addr != end);
>>  }
> 
> Well that's ugly.
> 
> We could redo unmap_kernel_range() so it takes an `atomic' flag then
> loops around unmapping N MB at a time, doing
> 
>       if (!atomic)
>               cond_resched()
> 
> 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.

David

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