From: Amir Goldstein <amir73il@gmail.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: linux-xfs <linux-xfs@vger.kernel.org>,
linux-f2fs-devel@lists.sourceforge.net,
Theodore Tso <tytso@mit.edu>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Artem Bityutskiy <dedekind1@gmail.com>,
Richard Weinberger <richard@nod.at>,
yuchao0@huawei.com, Hugh Dickins <hughd@google.com>,
Josef Bacik <josef@toxicpanda.com>,
Adrian Hunter <adrian.hunter@intel.com>,
Christoph Hellwig <hch@infradead.org>, Chris Mason <clm@fb.com>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Al Viro <viro@zeniv.linux.org.uk>,
linux-mtd@lists.infradead.org, Jan Kara <jack@suse.com>,
dsterba@suse.com, Jaegeuk Kim <jaegeuk@kernel.org>,
Ext4 <linux-ext4@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
Linux Btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH] vfs: don't decrement i_nlink in d_tmpfile
Date: Fri, 15 Feb 2019 10:04:12 +0200 [thread overview]
Message-ID: <CAOQ4uxho2AK7g-uhHykGaG6n+aqad-SaCTC6Z_EaA4Jn07tDSg@mail.gmail.com> (raw)
In-Reply-To: <20190214234908.GA6474@magnolia>
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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next prev parent reply other threads:[~2019-02-15 8:05 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-14 23:49 [PATCH] vfs: don't decrement i_nlink in d_tmpfile Darrick J. Wong
2019-02-15 8:04 ` Amir Goldstein [this message]
2019-02-15 15:56 ` Darrick J. Wong
2019-02-15 22:39 ` [PATCH v2] " Darrick J. Wong
2019-02-17 0:26 ` Al Viro
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