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

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

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.




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