linux-kernel - [PATCH 3.16 167/202] tmpfs: fix link accounting when a tmpfile is linked in

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Message-ID: <lsq.1556377989.782560896@decadent.org.uk>
Date:   Sat, 27 Apr 2019 16:13:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Matej Kupljen" <matej.kupljen@...il.com>,
        "Hugh Dickins" <hughd@...gle.com>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Subject: [PATCH 3.16 167/202] tmpfs: fix link accounting when a tmpfile is
 linked in

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

commit 1062af920c07f5b54cf5060fde3339da6df0cf6b upstream.

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@...cle.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Reported-by: Matej Kupljen <matej.kupljen@...il.com>
Acked-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/shmem.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2013,10 +2013,14 @@ 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;