shmem: update memory reservation on truncate

A shared anonymous mapping created without MAP_NORESERVE holds memory
reservation for whole range of shmem segment.  Usually there is no way
to change its size, but /proc/<pid>/map_files/...  (available if
CONFIG_CHECKPOINT_RESTORE=y) allows that.

This patch adjusts the memory reservation in shmem_setattr().
Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -149,6 +149,19 @@ static inline void shmem_unacct_size(unsigned long flags, loff_t size)
 		vm_unacct_memory(VM_ACCT(size));
 }
 
+static inline int shmem_reacct_size(unsigned long flags,
+		loff_t oldsize, loff_t newsize)
+{
+	if (!(flags & VM_NORESERVE)) {
+		if (VM_ACCT(newsize) > VM_ACCT(oldsize))
+			return security_vm_enough_memory_mm(current->mm,
+					VM_ACCT(newsize) - VM_ACCT(oldsize));
+		else if (VM_ACCT(newsize) < VM_ACCT(oldsize))
+			vm_unacct_memory(VM_ACCT(oldsize) - VM_ACCT(newsize));
+	}
+	return 0;
+}
+
 /*
  * ... whereas tmpfs objects are accounted incrementally as
  * pages are allocated, in order to allow huge sparse files.
@@ -549,6 +562,10 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 		loff_t newsize = attr->ia_size;
 
 		if (newsize != oldsize) {
+			error = shmem_reacct_size(SHMEM_I(inode)->flags,
+					oldsize, newsize);
+			if (error)
+				return error;
 			i_size_write(inode, newsize);
 			inode->i_ctime = inode->i_mtime = CURRENT_TIME;
 		}