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Wong" To: Amir Goldstein Subject: Re: [PATCH] vfs: don't decrement i_nlink in d_tmpfile Message-ID: <20190215155604.GL32253@magnolia> References: <20190214234908.GA6474@magnolia> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9168 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1902150109 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190215_075631_641945_6309BAE2 X-CRM114-Status: GOOD ( 30.66 ) X-BeenThere: linux-mtd@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-xfs , linux-f2fs-devel@lists.sourceforge.net, Theodore Tso , linux-fsdevel , Artem Bityutskiy , Richard Weinberger , yuchao0@huawei.com, Hugh Dickins , Josef Bacik , Adrian Hunter , Christoph Hellwig , Chris Mason , Andreas Dilger , Al Viro , linux-mtd@lists.infradead.org, Jan Kara , dsterba@suse.com, Jaegeuk Kim , Ext4 , Linux MM , Linux Btrfs Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-mtd" Errors-To: linux-mtd-bounces+linux-mtd=archiver.kernel.org@lists.infradead.org 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 wrote: > > > > From: Darrick J. Wong > > > > 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 > > --- > > 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. ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/