[PATCH] vfs: don't decrement i_nlink in d_tmpfile

[PATCH] vfs: don't decrement i_nlink in d_tmpfile

[Darrick J. Wong]

From: Darrick J. Wong <darrick.wong@oracle.com>

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@oracle.com>
---
 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);
 	spin_lock(&dentry->d_parent->d_lock);
 	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 	dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
diff --git a/fs/ext2/namei.c b/fs/ext2/namei.c
index 0c26dcc5d850..8542e9ce9677 100644
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -117,7 +117,7 @@ static int ext2_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		return PTR_ERR(inode);
 
 	ext2_set_file_ops(inode);
-	mark_inode_dirty(inode);
+	inode_dec_link_count(inode);
 	d_tmpfile(dentry, inode);
 	unlock_new_inode(inode);
 	return 0;
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 2b928eb07fa2..7502432f9816 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2517,6 +2517,7 @@ static int ext4_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		inode->i_op = &ext4_file_inode_operations;
 		inode->i_fop = &ext4_file_operations;
 		ext4_set_aops(inode);
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 		err = ext4_orphan_add(handle, inode);
 		if (err)
diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
index 62d9829f3a6a..31a556af5f3a 100644
--- a/fs/f2fs/namei.c
+++ b/fs/f2fs/namei.c
@@ -780,6 +780,7 @@ static int __f2fs_tmpfile(struct inode *dir, struct dentry *dentry,
 		f2fs_i_links_write(inode, false);
 		*whiteout = inode;
 	} else {
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	/* link_count was changed by d_tmpfile as well. */
diff --git a/fs/minix/namei.c b/fs/minix/namei.c
index 1a6084d2b02e..3249f86c476a 100644
--- a/fs/minix/namei.c
+++ b/fs/minix/namei.c
@@ -57,7 +57,7 @@ static int minix_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 	struct inode *inode = minix_new_inode(dir, mode, &error);
 	if (inode) {
 		minix_set_inode(inode, 0);
-		mark_inode_dirty(inode);
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	return error;
diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index 5767b373a8ff..7187e4fd7561 100644
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -419,6 +419,7 @@ static int do_tmpfile(struct inode *dir, struct dentry *dentry,
 		drop_nlink(inode);
 		*whiteout = inode;
 	} else {
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	ubifs_assert(c, ui->dirty);
diff --git a/fs/udf/namei.c b/fs/udf/namei.c
index 58cc2414992b..38bd021f9673 100644
--- a/fs/udf/namei.c
+++ b/fs/udf/namei.c
@@ -652,7 +652,7 @@ static int udf_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		inode->i_data.a_ops = &udf_aops;
 	inode->i_op = &udf_file_inode_operations;
 	inode->i_fop = &udf_file_operations;
-	mark_inode_dirty(inode);
+	inode_dec_link_count(inode);
 	d_tmpfile(dentry, inode);
 	unlock_new_inode(inode);
 	return 0;
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 1efef69a7f1c..f48ffd7a8d3e 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -191,18 +191,9 @@ xfs_generic_create(
 
 	xfs_setup_iops(ip);
 
-	if (tmpfile) {
-		/*
-		 * The VFS requires that any inode fed to d_tmpfile must have
-		 * nlink == 1 so that it can decrement the nlink in d_tmpfile.
-		 * However, we created the temp file with nlink == 0 because
-		 * we're not allowed to put an inode with nlink > 0 on the
-		 * unlinked list.  Therefore we have to set nlink to 1 so that
-		 * d_tmpfile can immediately set it back to zero.
-		 */
-		set_nlink(inode, 1);
+	if (tmpfile)
 		d_tmpfile(dentry, inode);
-	} else
+	else
 		d_instantiate(dentry, inode);
 
 	xfs_finish_inode_setup(ip);
diff --git a/mm/shmem.c b/mm/shmem.c
index 6ece1e2fe76e..4a7810093561 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2818,6 +2818,7 @@ shmem_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		error = simple_acl_create(dir, inode);
 		if (error)
 			goto out_iput;
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	return error;

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Re: [PATCH] vfs: don't decrement i_nlink in d_tmpfile

[Amir Goldstein]

On Fri, Feb 15, 2019 at 4:23 AM Darrick J. Wong <darrick.wong@oracle.com> wrote:
>
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> 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@oracle.com>
> ---
>  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.

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Re: [PATCH] vfs: don't decrement i_nlink in d_tmpfile

[Darrick J. Wong]

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@oracle.com> wrote:
> >
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> >
> > 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@oracle.com>
> > ---
> >  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.

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[PATCH v2] vfs: don't decrement i_nlink in d_tmpfile

[Darrick J. Wong]

From: Darrick J. Wong <darrick.wong@oracle.com>

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, 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@oracle.com>
---
v2: convert BUG_ON to WARN_ON per review comment
---
 fs/btrfs/inode.c  |    8 --------
 fs/dcache.c       |   10 ++++++----
 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(+), 26 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..bb349d423055 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -3042,12 +3042,14 @@ 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));
+	WARN_ON(dentry->d_name.name != dentry->d_iname ||
+		!d_unlinked(dentry) || inode->i_nlink != 0);
 	spin_lock(&dentry->d_parent->d_lock);
 	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 	dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
diff --git a/fs/ext2/namei.c b/fs/ext2/namei.c
index 0c26dcc5d850..8542e9ce9677 100644
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -117,7 +117,7 @@ static int ext2_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		return PTR_ERR(inode);
 
 	ext2_set_file_ops(inode);
-	mark_inode_dirty(inode);
+	inode_dec_link_count(inode);
 	d_tmpfile(dentry, inode);
 	unlock_new_inode(inode);
 	return 0;
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 2b928eb07fa2..7502432f9816 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2517,6 +2517,7 @@ static int ext4_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		inode->i_op = &ext4_file_inode_operations;
 		inode->i_fop = &ext4_file_operations;
 		ext4_set_aops(inode);
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 		err = ext4_orphan_add(handle, inode);
 		if (err)
diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
index 62d9829f3a6a..31a556af5f3a 100644
--- a/fs/f2fs/namei.c
+++ b/fs/f2fs/namei.c
@@ -780,6 +780,7 @@ static int __f2fs_tmpfile(struct inode *dir, struct dentry *dentry,
 		f2fs_i_links_write(inode, false);
 		*whiteout = inode;
 	} else {
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	/* link_count was changed by d_tmpfile as well. */
diff --git a/fs/minix/namei.c b/fs/minix/namei.c
index 1a6084d2b02e..3249f86c476a 100644
--- a/fs/minix/namei.c
+++ b/fs/minix/namei.c
@@ -57,7 +57,7 @@ static int minix_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 	struct inode *inode = minix_new_inode(dir, mode, &error);
 	if (inode) {
 		minix_set_inode(inode, 0);
-		mark_inode_dirty(inode);
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	return error;
diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index 5767b373a8ff..7187e4fd7561 100644
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -419,6 +419,7 @@ static int do_tmpfile(struct inode *dir, struct dentry *dentry,
 		drop_nlink(inode);
 		*whiteout = inode;
 	} else {
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	ubifs_assert(c, ui->dirty);
diff --git a/fs/udf/namei.c b/fs/udf/namei.c
index 58cc2414992b..38bd021f9673 100644
--- a/fs/udf/namei.c
+++ b/fs/udf/namei.c
@@ -652,7 +652,7 @@ static int udf_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		inode->i_data.a_ops = &udf_aops;
 	inode->i_op = &udf_file_inode_operations;
 	inode->i_fop = &udf_file_operations;
-	mark_inode_dirty(inode);
+	inode_dec_link_count(inode);
 	d_tmpfile(dentry, inode);
 	unlock_new_inode(inode);
 	return 0;
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 1efef69a7f1c..f48ffd7a8d3e 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -191,18 +191,9 @@ xfs_generic_create(
 
 	xfs_setup_iops(ip);
 
-	if (tmpfile) {
-		/*
-		 * The VFS requires that any inode fed to d_tmpfile must have
-		 * nlink == 1 so that it can decrement the nlink in d_tmpfile.
-		 * However, we created the temp file with nlink == 0 because
-		 * we're not allowed to put an inode with nlink > 0 on the
-		 * unlinked list.  Therefore we have to set nlink to 1 so that
-		 * d_tmpfile can immediately set it back to zero.
-		 */
-		set_nlink(inode, 1);
+	if (tmpfile)
 		d_tmpfile(dentry, inode);
-	} else
+	else
 		d_instantiate(dentry, inode);
 
 	xfs_finish_inode_setup(ip);
diff --git a/mm/shmem.c b/mm/shmem.c
index 6ece1e2fe76e..4a7810093561 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2818,6 +2818,7 @@ shmem_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 		error = simple_acl_create(dir, inode);
 		if (error)
 			goto out_iput;
+		inode_dec_link_count(inode);
 		d_tmpfile(dentry, inode);
 	}
 	return error;

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Re: [PATCH v2] vfs: don't decrement i_nlink in d_tmpfile

[Al Viro]

On Fri, Feb 15, 2019 at 02:39:25PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> 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, 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.

NAK.  You are changing semantics of existing helper, requiring to add
boilerplate to existing users.  With zero indication that such need
has appeared - no warnings, etc.

If you need a variant that wouldn't do nlink decrement, just add it
and turn the existing one into a wrapper.  Yield smaller patch, at that...

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