vunmap() on large regions may trigger soft lockup warnings

[David Vrabel <david.vrabel@citrix.com>]

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@citrix.com>

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.

Reported-by: Dietmar Hahn <dietmar.hahn@ts.fujitsu.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 mm/vmalloc.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 0fdf968..b1b5b39 100644
--- 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);
 }

-- 
1.7.2.5