[RFC][PATCH 1/2] vmalloc: reorder unmap and removal entry

[KAMEZAWA Hiroyuki]

> I wonder fs/proc/kcore.c's vmalloc area access needs some fix. let me try.
> 
> Thanks,
Finally, I wrote 2 patches. maybe 2/2 is a fix for the bug.
but needs comments.

Comaparing 2 calls, vfree() and vread()
==
vfree()
  -> __vumap()
     -> remove_vm_area()
       -> free_unmap_vmap_area         #
	 -> try_purge_vmap_area_lazy() #
           ->__purge_vmap_area_lazy(); #
             -> unmap_vmap_area()      # unmap memory here.
       -> write_lock(&vmlist_lock)
          remvoe vm_struct from list
          write_unlock(&vmlist_lock).
==
vread()
 -> read_lock(&vmlist_lock);
    get vm_struct -> do memcpy
    read_unlock(&vmlist_lock);
==

Hmm, maybe not related to original bug but above order should be fixed.

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

vmap area should be purged after vm_struct is removed from the list
because vread/vwrite etc...believes the range is valid while it's on
vm_struct list.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/vmalloc.c |   14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

Index: linux-2.6.31-rc4/mm/vmalloc.c
===================================================================
--- linux-2.6.31-rc4.orig/mm/vmalloc.c
+++ linux-2.6.31-rc4/mm/vmalloc.c
@@ -1256,17 +1256,21 @@ struct vm_struct *remove_vm_area(const v
 	if (va && va->flags & VM_VM_AREA) {
 		struct vm_struct *vm = va->private;
 		struct vm_struct *tmp, **p;
-
-		vmap_debug_free_range(va->va_start, va->va_end);
-		free_unmap_vmap_area(va);
-		vm->size -= PAGE_SIZE;
-
+		/*
+		 * remove from list and disallow access to this vm_struct
+		 * before unmap. (address range confliction is maintained by
+		 * vmap.)
+		 */
 		write_lock(&vmlist_lock);
 		for (p = &vmlist; (tmp = *p) != vm; p = &tmp->next)
 			;
 		*p = tmp->next;
 		write_unlock(&vmlist_lock);
 
+		vmap_debug_free_range(va->va_start, va->va_end);
+		free_unmap_vmap_area(va);
+		vm->size -= PAGE_SIZE;
+
 		return vm;
 	}
 	return NULL;

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