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    Subject[PATCH AUTOSEL 4.20 37/60] tmpfs: fix link accounting when a tmpfile is linked in
    Date
    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 5d07e0b1352f..7872e3b75e57 100644
    --- a/mm/shmem.c
    +++ b/mm/shmem.c
    @@ -2852,10 +2852,14 @@ 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);
    --
    2.19.1
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