On Fri, Feb 15, 2019 at 10:04:12AM +0200, Amir Goldstein wrote:
> 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().
Fair enough, I'll remove the redundant checks and downgrade that to a
WARN_ON, if nobody else objects....
--D
> Thanks,
> Amir.
