[PATCH] tmpfs: fix link accounting when a tmpfile is linked in

[Hugh Dickins <hughd@google.com>]

From: "Darrick J. Wong" <darrick.wong@oracle.com>

tmpfs has a peculiarity of accounting hard links as if they were separate
inodes: so that when the number of inodes is limited, as it is by default,
a user cannot soak up an unlimited amount of unreclaimable dcache memory
just by repeatedly linking a file.

But when v3.11 added O_TMPFILE, and the ability to use linkat() on the fd,
we missed accommodating this new case in tmpfs: "df -i" shows that an
extra "inode" remains accounted after the file is unlinked and the fd
closed and the actual inode evicted.  If a user repeatedly links tmpfiles
into a tmpfs, the limit will be hit (ENOSPC) even after they are deleted.

Just skip the extra reservation from shmem_link() in this case: there's
a sense in which this first link of a tmpfile is then cheaper than a
hard link of another file, but the accounting works out, and there's
still good limiting, so no need to do anything more complicated.

Fixes: f4e0c30c191 ("allow the temp files created by open() to be linked to")
Reported-by: Matej Kupljen <matej.kupljen@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/shmem.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- 5.0-rc7/mm/shmem.c	2019-01-06 19:15:45.764805103 -0800
+++ linux/mm/shmem.c	2019-02-18 13:56:48.388032606 -0800
@@ -2854,10 +2854,14 @@ static int shmem_link(struct dentry *old
 	 * No ordinary (disk based) filesystem counts links as inodes;
 	 * but each new link needs a new dentry, pinning lowmem, and
 	 * tmpfs dentries cannot be pruned until they are unlinked.
+	 * But if an O_TMPFILE file is linked into the tmpfs, the
+	 * first link must skip that, to get the accounting right.
 	 */
-	ret = shmem_reserve_inode(inode->i_sb);
-	if (ret)
-		goto out;
+	if (inode->i_nlink) {
+		ret = shmem_reserve_inode(inode->i_sb);
+		if (ret)
+			goto out;
+	}
 
 	dir->i_size += BOGO_DIRENT_SIZE;
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);