From: "Darrick J. Wong" darrick.wong@oracle.com
[ 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 --- mm/shmem.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c index b6cf0e8e685b..bf13966b009c 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2901,10 +2901,14 @@ 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);