[PATCH] mm: sync vmalloc address space page tables in alloc_vm_area()

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

From: David Vrabel <david.vrabel@citrix.com>

Xen backend drivers (e.g., blkback and netback) would sometimes fail
to map grant pages into the vmalloc address space allocated with
alloc_vm_area().  The GNTTABOP_map_grant_ref would fail because Xen
could not find the page (in the L2 table) containing the PTEs it
needed to update.

(XEN) mm.c:3846:d0 Could not find L1 PTE for address fbb42000

netback and blkback were making the hypercall from a kernel thread
where task->active_mm != &init_mm and alloc_vm_area() was only
updating the page tables for init_mm.  The usual method of deferring
the update to the page tables of other processes (i.e., after taking a
fault) doesn't work as a fault cannot occur during the hypercall.

This would work on some systems depending on what else was using
vmalloc.

Fix this by reverting ef691947d8a3d479e67652312783aedcf629320a
(vmalloc: remove vmalloc_sync_all() from alloc_vm_area()) and add a
comment to explain why it's needed.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
 mm/vmalloc.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7ef0903..5016f19 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2140,6 +2140,14 @@ struct vm_struct *alloc_vm_area(size_t size)
 		return NULL;
 	}
 
+	/*
+	 * If the allocated address space is passed to a hypercall
+	 * before being used then we cannot rely on a page fault to
+	 * trigger an update of the page tables.  So sync all the page
+	 * tables here.
+	 */
+	vmalloc_sync_all();
+
 	return area;
 }
 EXPORT_SYMBOL_GPL(alloc_vm_area);
-- 
1.7.2.5