mm: reintroduce and deprecate rlimit based access for SHM_HUGETLB

Allow non root users with sufficient mlock rlimits to be able to allocate
hugetlb backed shm for now.  Deprecate this though.  This is being
deprecated because the mlock based rlimit checks for SHM_HUGETLB is not
consistent with mmap based huge page allocations.
Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Reviewed-by: Mel Gorman <mel@csn.ul.ie>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: Adam Litke <agl@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/feature-removal-schedule.txt

@@ -311,6 +311,18 @@ Who:	Vlad Yasevich <vladislav.yasevich@hp.com>
 
 ---------------------------
 
+What:	Ability for non root users to shm_get hugetlb pages based on mlock
+	resource limits
+When:	2.6.31
+Why:	Non root users need to be part of /proc/sys/vm/hugetlb_shm_group or
+	have CAP_IPC_LOCK to be able to allocate shm segments backed by
+	huge pages.  The mlock based rlimit check to allow shm hugetlb is
+	inconsistent with mmap based allocations.  Hence it is being
+	deprecated.
+Who:	Ravikiran Thirumalai <kiran@scalex86.org>
+
+---------------------------
+
 What:	CONFIG_THERMAL_HWMON
 When:	January 2009
 Why:	This option was introduced just to allow older lm-sensors userspace

fs/hugetlbfs/inode.c

@@ -949,6 +949,7 @@ static int can_do_hugetlb_shm(void)
 struct file *hugetlb_file_setup(const char *name, size_t size, int acctflag)
 {
 	int error = -ENOMEM;
+	int unlock_shm = 0;
 	struct file *file;
 	struct inode *inode;
 	struct dentry *dentry, *root;
@@ -958,8 +959,14 @@ struct file *hugetlb_file_setup(const char *name, size_t size, int acctflag)
 	if (!hugetlbfs_vfsmount)
 		return ERR_PTR(-ENOENT);
 
-	if (!can_do_hugetlb_shm())
-		return ERR_PTR(-EPERM);
+	if (!can_do_hugetlb_shm()) {
+		if (user_shm_lock(size, user)) {
+			unlock_shm = 1;
+			WARN_ONCE(1,
+			  "Using mlock ulimits for SHM_HUGETLB deprecated\n");
+		} else
+			return ERR_PTR(-EPERM);
+	}
 
 	root = hugetlbfs_vfsmount->mnt_root;
 	quick_string.name = name;
@@ -999,6 +1006,8 @@ out_inode:
 out_dentry:
 	dput(dentry);
 out_shm_unlock:
+	if (unlock_shm)
+		user_shm_unlock(size, user);
 	return ERR_PTR(error);
 }