Re: [RFC/PATCH] No get_user/put_user while holding mmap_sem in do_pages_stat?

[Christoph Lameter <cl@linux-foundation.org>]

I guess the simplest solution would be to move the taking of mmap_sem into
the loop. Mean taking mmap_sem for every page that we determine the status of.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c	2008-12-09 08:07:08.796603952 -0600
+++ linux-2.6/mm/migrate.c	2008-12-09 08:08:36.400116263 -0600
@@ -994,8 +994,6 @@
 	unsigned long i;
 	int err;

-	down_read(&mm->mmap_sem);
-
 	for (i = 0; i < nr_pages; i++) {
 		const void __user *p;
 		unsigned long addr;
@@ -1007,12 +1005,17 @@
 			goto out;
 		addr = (unsigned long) p;

+		down_read(&mm->mmap_sem);
+
 		vma = find_vma(mm, addr);
-		if (!vma)
+		if (!vma) {
+			up_read(&mm->mmap_sem);
 			goto set_status;
-
+		}
 		page = follow_page(vma, addr, 0);

+		up_read(&mm->mmap_sem);
+
 		err = PTR_ERR(page);
 		if (IS_ERR(page))
 			goto set_status;
@@ -1029,7 +1032,6 @@
 	err = 0;

 out:
-	up_read(&mm->mmap_sem);
 	return err;
 }