[PATCH v10 13/17] mm: vmalloc: add flag preventing guard hole allocation

[Andrey Ryabinin]

For instrumenting global variables KASan will shadow memory
backing memory for modules. So on module loading we will need
to allocate shadow memory and map it at exact virtual address.
__vmalloc_node_range() seems like the best fit for that purpose,
except it puts a guard hole after allocated area.

Add a new vm_struct flag 'VM_NO_GUARD' indicating that vm area
doesn't have a guard hole.

Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
---
 include/linux/vmalloc.h | 9 +++++++--
 mm/vmalloc.c            | 6 ++----
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index b87696f..1526fe7 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -16,6 +16,7 @@ struct vm_area_struct;		/* vma defining user mapping in mm_types.h */
 #define VM_USERMAP		0x00000008	/* suitable for remap_vmalloc_range */
 #define VM_VPAGES		0x00000010	/* buffer for pages was vmalloc'ed */
 #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully initialized */
+#define VM_NO_GUARD		0x00000040      /* don't add guard page */
 /* bits [20..32] reserved for arch specific ioremap internals */
 
 /*
@@ -96,8 +97,12 @@ void vmalloc_sync_all(void);
 
 static inline size_t get_vm_area_size(const struct vm_struct *area)
 {
-	/* return actual size without guard page */
-	return area->size - PAGE_SIZE;
+	if (!(area->flags & VM_NO_GUARD))
+		/* return actual size without guard page */
+		return area->size - PAGE_SIZE;
+	else
+		return area->size;
+
 }
 
 extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 39c3388..2e74e99 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1324,10 +1324,8 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
 	if (unlikely(!area))
 		return NULL;
 
-	/*
-	 * We always allocate a guard page.
-	 */
-	size += PAGE_SIZE;
+	if (!(flags & VM_NO_GUARD))
+		size += PAGE_SIZE;
 
 	va = alloc_vmap_area(size, align, start, end, node, gfp_mask);
 	if (IS_ERR(va)) {
-- 
2.2.2

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