tmpfs: fix link accounting when a tmpfile is linked in

[ Upstream commit 1062af920c07f5b54cf5060fde3339da6df0cf6b ]

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.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1902182134370.7035@eggly.anvils
Fixes: f4e0c30c191 ("allow the temp files created by open() to be linked to")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Matej Kupljen <matej.kupljen@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 6c10f1d922517984c8d31fdc92c42c554ea19fec..9b78c04f532b3e90c57a44a32a4b7218930db69d 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3102,10 +3102,14 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
         * 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);