On Fri, Feb 15, 2019 at 4:23 AM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
>
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> d_tmpfile was introduced to instantiate an inode in the dentry cache as
> a temporary file. This helper decrements the inode's nlink count and
> dirties the inode, presumably so that filesystems could call new_inode
> to create a new inode with nlink == 1 and then call d_tmpfile which will
> decrement nlink.
>
> However, this doesn't play well with XFS, which needs to allocate,
> initialize, and insert a tempfile inode on its unlinked list in a single
> transaction. In order to maintain referential integrity of the XFS
> metadata, we cannot have an inode on the unlinked list with nlink >= 1.
>
> XFS and btrfs hack around d_tmpfile's behavior by creating the inode
> with nlink == 0 and then incrementing it just prior to calling
> d_tmpfile, anticipating that it will be reset to 0.
>
> Everywhere else outside of d_tmpfile, it appears that nlink updates and
> persistence is the responsibility of individual filesystems. Therefore,
> move the nlink decrement out of d_tmpfile into the callers, and require
> that callers only pass in inodes with nlink already set to 0.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> fs/btrfs/inode.c | 8 --------
> fs/dcache.c | 8 ++++++--
> fs/ext2/namei.c | 2 +-
> fs/ext4/namei.c | 1 +
> fs/f2fs/namei.c | 1 +
> fs/minix/namei.c | 2 +-
> fs/ubifs/dir.c | 1 +
> fs/udf/namei.c | 2 +-
> fs/xfs/xfs_iops.c | 13 ++-----------
> mm/shmem.c | 1 +
> 10 files changed, 15 insertions(+), 24 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 5c349667c761..bd189fc50f83 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -10382,14 +10382,6 @@ static int btrfs_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
> if (ret)
> goto out;
>
> - /*
> - * We set number of links to 0 in btrfs_new_inode(), and here we set
> - * it to 1 because d_tmpfile() will issue a warning if the count is 0,
> - * through:
> - *
> - * d_tmpfile() -> inode_dec_link_count() -> drop_nlink()
> - */
> - set_nlink(inode, 1);
> d_tmpfile(dentry, inode);
> unlock_new_inode(inode);
> mark_inode_dirty(inode);
> diff --git a/fs/dcache.c b/fs/dcache.c
> index aac41adf4743..5fb4ecce2589 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -3042,12 +3042,16 @@ void d_genocide(struct dentry *parent)
>
> EXPORT_SYMBOL(d_genocide);
>
> +/*
> + * Instantiate an inode in the dentry cache as a temporary file. Callers must
> + * ensure that @inode has a zero link count.
> + */
> void d_tmpfile(struct dentry *dentry, struct inode *inode)
> {
> - inode_dec_link_count(inode);
> BUG_ON(dentry->d_name.name != dentry->d_iname ||
> !hlist_unhashed(&dentry->d_u.d_alias) ||
> - !d_unlinked(dentry));
> + !d_unlinked(dentry) ||
> + inode->i_nlink != 0);
You've just promoted i_nlink filesystem accounting error (which
are not that rare) from WARN_ON() to BUG_ON(), not to mention
Linus' objection to any use of BUG_ON() at all.
!hlist_unhashed is anyway checked again in d_instantiate().
!d_unlinked is not a reason to break the machine.
The name check is really not a reason to break the machine.
Can probably make tmp name code conditional to WARN_ON().
Thanks,
Amir.
