[PATCH RESEND] nfs: prevent truncate on active swapfile

[Omar Sandoval <osandov@osandov.com>]

Most filesystems prevent truncation of an active swapfile by way of
inode_newsize_ok, called from inode_change_ok. NFS doesn't call either
from nfs_setattr, presumably because most of these checks are expected
to be done server-side. However, the IS_SWAPFILE check can only be done
client-side, and truncating a swapfile can't possibly be good.

Signed-off-by: Omar Sandoval <osandov@osandov.com>
---
Hi, Trond,

Now that the holidays are over, could you take a look at this? It was
generated against v3.19-rc3.

Thanks!

 fs/nfs/inode.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 4bffe63..9205513 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -506,10 +506,15 @@ nfs_setattr(struct dentry *dentry, struct iattr *attr)
 		attr->ia_valid &= ~ATTR_MODE;
 
 	if (attr->ia_valid & ATTR_SIZE) {
+		loff_t i_size;
+
 		BUG_ON(!S_ISREG(inode->i_mode));
 
-		if (attr->ia_size == i_size_read(inode))
+		i_size = i_size_read(inode);
+		if (attr->ia_size == i_size)
 			attr->ia_valid &= ~ATTR_SIZE;
+		else if (attr->ia_size < i_size && IS_SWAPFILE(inode))
+			return -ETXTBSY;
 	}
 
 	/* Optimization: if the end result is no change, don't RPC */
-- 
2.2.1