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

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 ef691947d8 ("vmalloc: remove vmalloc_sync_all()
from alloc_vm_area()") and add a comment to explain why it's needed.

Signed-off-by: David Vrabel <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Keir Fraser <[email protected]>
Cc: <[email protected]>		[3.0.x]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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);