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tmpfs: fix link accounting when a tmpfile is linked in

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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: f4e0c30c19 ("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>
This commit is contained in:
Darrick J. Wong 2019-02-21 08:48:09 -08:00 committed by Linus Torvalds
parent 4e37504d1c
commit 1062af920c
1 changed files with 7 additions and 3 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

4
mm/shmem.c
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@ -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.
*/
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);