tmpfs: fix link accounting when a tmpfile is linked in [Linux 5.0]

tmpfs: fix link accounting when a tmpfile is linked in [Linux 5.0]

This Linux kernel change "tmpfs: fix link accounting when a tmpfile is linked in" is included in the Linux 5.0 release. This change is authored by Darrick J. Wong <darrick.wong [at] oracle.com> on Thu Feb 21 08:48:09 2019 -0800. The commit for this change in Linux stable tree is 1062af9 (patch).

tmpfs: fix link accounting when a tmpfile is linked in

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/[email protected]
Fixes: f4e0c30c191 ("allow the temp files created by open() to be linked to")
Signed-off-by: Darrick J. Wong <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Reported-by: Matej Kupljen <[email protected]>
Acked-by: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

There are 10 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

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

diff --git a/mm/shmem.c b/mm/shmem.c
index 6ece1e2..0905215 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2854,10 +2854,14 @@ 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);

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