Re: [Ocfs2-devel] [PATCH 2/7] vfs: flush and wait for io when setting the immutable flag via SETFLAGS

[Darrick J. Wong]

On Mon, Jun 24, 2019 at 02:58:17PM -0700, Darrick J. Wong wrote:
> On Mon, Jun 24, 2019 at 01:37:37PM +0200, Jan Kara wrote:
> > On Fri 21-06-19 16:57:07, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > > 
> > > When we're using FS_IOC_SETFLAGS to set the immutable flag on a file, we
> > > need to ensure that userspace can't continue to write the file after the
> > > file becomes immutable.  To make that happen, we have to flush all the
> > > dirty pagecache pages to disk to ensure that we can fail a page fault on
> > > a mmap'd region, wait for pending directio to complete, and hope the
> > > caller locked out any new writes by holding the inode lock.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > 
> > Seeing the way this worked out, is there a reason to have separate
> > vfs_ioc_setflags_flush_data() instead of folding the functionality in
> > vfs_ioc_setflags_check() (possibly renaming it to
> > vfs_ioc_setflags_prepare() to indicate it does already some changes)? I
> > don't see any place that would need these two separated...
> 
> XFS needs them to be separated.
> 
> If we even /think/ that we're going to be setting the immutable flag
> then we need to grab the IOLOCK and the MMAPLOCK to prevent further
> writes while we drain all the directio writes and dirty data.  IO
> completions for the write draining can take the ILOCK, which means that
> we can't have grabbed it yet.
> 
> Next, we grab the ILOCK so we can check the new flags against the inode
> and then update the inode core.
> 
> For most filesystems I think it suffices to inode_lock and then do both,
> though.

Heh, lol, that applies to fssetxattr, not to setflags, because xfs
setflags implementation open-codes the relevant fssetxattr pieces.
So for setflags we can combine both parts into a single _prepare
function.

--D

> > > +/*
> > > + * Flush all pending IO and dirty mappings before setting S_IMMUTABLE on an
> > > + * inode via FS_IOC_SETFLAGS.  If the flush fails we'll clear the flag before
> > > + * returning error.
> > > + *
> > > + * Note: the caller should be holding i_mutex, or else be sure that
> > > + * they have exclusive access to the inode structure.
> > > + */
> > > +static inline int vfs_ioc_setflags_flush_data(struct inode *inode, int flags)
> > > +{
> > > +	int ret;
> > > +
> > > +	if (!vfs_ioc_setflags_need_flush(inode, flags))
> > > +		return 0;
> > > +
> > > +	inode_set_flags(inode, S_IMMUTABLE, S_IMMUTABLE);
> > > +	ret = inode_flush_data(inode);
> > > +	if (ret)
> > > +		inode_set_flags(inode, 0, S_IMMUTABLE);
> > > +	return ret;
> > > +}
> > 
> > Also this sets S_IMMUTABLE whenever vfs_ioc_setflags_need_flush() returns
> > true. That is currently the right thing but seems like a landmine waiting
> > to trip? So I'd just drop the vfs_ioc_setflags_need_flush() abstraction to
> > make it clear what's going on.
> 
> Ok.
> 
> --D
> 
> > 
> > 								Honza
> > -- 
> > Jan Kara <jack@xxxxxxxx>
> > SUSE Labs, CR
> 
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