Re: [PATCH v1 11/17] xfs: add parent attributes to link

From: Alli <allison.henderson@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH v1 11/17] xfs: add parent attributes to link
Date: Fri, 17 Jun 2022 17:32:47 -0700	[thread overview]
Message-ID: <37baf16c95601e8919ebd1ecda704084cb121148.camel@oracle.com> (raw)
In-Reply-To: <20220616223923.GG227878@dread.disaster.area>

On Fri, 2022-06-17 at 08:39 +1000, Dave Chinner wrote:
> On Sat, Jun 11, 2022 at 02:41:54AM -0700, Allison Henderson wrote:
> > This patch modifies xfs_link to add a parent pointer to the inode.
> > 
> > [bfoster: rebase, use VFS inode fields, fix xfs_bmap_finish()
> > usage]
> > [achender: rebased, changed __unint32_t to xfs_dir2_dataptr_t,
> >            fixed null pointer bugs]
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> > ---
> >  fs/xfs/xfs_inode.c | 78 ++++++++++++++++++++++++++++++++++++----
> > ------
> >  fs/xfs/xfs_trans.c |  7 +++--
> >  fs/xfs/xfs_trans.h |  2 +-
> >  3 files changed, 67 insertions(+), 20 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index 6b1e4cb11b5c..41c58df8e568 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -1254,14 +1254,28 @@ xfs_create_tmpfile(
> >  
> >  int
> >  xfs_link(
> > -	xfs_inode_t		*tdp,
> > -	xfs_inode_t		*sip,
> > -	struct xfs_name		*target_name)
> > -{
> > -	xfs_mount_t		*mp = tdp->i_mount;
> > -	xfs_trans_t		*tp;
> > -	int			error, nospace_error = 0;
> > -	int			resblks;
> > +	xfs_inode_t			*tdp,
> > +	xfs_inode_t			*sip,
> > +	struct xfs_name			*target_name)
> > +{
> > +	xfs_mount_t			*mp = tdp->i_mount;
> > +	xfs_trans_t			*tp;
> > +	int				error, nospace_error = 0;
> > +	int				resblks;
> > +	struct xfs_parent_name_rec	rec;
> > +	xfs_dir2_dataptr_t		diroffset;
> > +
> > +	struct xfs_da_args		args = {
> > +		.dp		= sip,
> > +		.geo		= mp->m_attr_geo,
> > +		.whichfork	= XFS_ATTR_FORK,
> > +		.attr_filter	= XFS_ATTR_PARENT,
> > +		.op_flags	= XFS_DA_OP_OKNOENT,
> > +		.name		= (const uint8_t *)&rec,
> > +		.namelen	= sizeof(rec),
> > +		.value		= (void *)target_name->name,
> > +		.valuelen	= target_name->len,
> > +	};
> 
> Now that I've had a bit of a think about this, this pattern of
> placing the rec on the stack and then using it as a buffer that is
> then accessed in xfs_tran_commit() processing feels like a landmine.
> 
> That is, we pass transaction contexts around functions as they are
> largely independent constructs, but adding this stack variable to
> the defer ops attached to the transaction means that the transaction
> cannot be passed back to a caller for it to be committed - that will
> corrupt the stack buffer and hence silently corrupt the parent attr
> that is going to be logged when the transaction is finally committed.
> 
> Hence I think this needs to be wrapped up as a dynamically allocated
> structure that is freed when the defer ops are done with it. e.g.
> 
> struct xfs_parent_defer {
> 	struct xfs_parent_name_rec	rec;
> 	xfs_dir2_dataptr_t		diroffset;
> 	struct xfs_da_args		args;
> };
> 
> and then here:
> 
> >  
> >  	trace_xfs_link(tdp, target_name);
> >  
> > @@ -1278,11 +1292,17 @@ xfs_link(
> >  	if (error)
> >  		goto std_return;
> >  
> > +	if (xfs_has_larp(mp)) {
> > +		error = xfs_attr_grab_log_assist(mp);
> > +		if (error)
> > +			goto std_return;
> > +	}
> 
> 	struct xfs_parent_defer		*parent = NULL;
> .....
> 
> 	error = xfs_parent_init(mp, target_name, &parent);
> 	if (error)
> 		goto std_return;
> 
> and xfs_parent_init() looks something like this:
> 
> int
> xfs_parent_init(
> 	.....
> 	struct xfs_parent_defer		**parentp)
> {
> 	struct xfs_parent_defer		*parent;
> 
> 	if (!xfs_has_parent_pointers(mp))
> 		return 0;
> 
> 	error = xfs_attr_grab_log_assist(mp);
> 	if (error)
> 		return error;
> 
> 	parent = kzalloc(sizeof(*parent), GFP_KERNEL);
> 	if (!parent)
> 		return -ENOMEM;
> 
> 	/* init parent da_args */
> 
> 	*parentp = parent;
> 	return 0;
> }
> 
> With that in place, we then can wrap all this up:
> 
> >  
> > +	/*
> > +	 * If we have parent pointers, we now need to add the parent
> > record to
> > +	 * the attribute fork of the inode. If this is the initial
> > parent
> > +	 * attribute, we need to create it correctly, otherwise we can
> > just add
> > +	 * the parent to the inode.
> > +	 */
> > +	if (xfs_sb_version_hasparent(&mp->m_sb)) {
> > +		args.trans = tp;
> > +		xfs_init_parent_name_rec(&rec, tdp, diroffset);
> > +		args.hashval = xfs_da_hashname(args.name,
> > +					       args.namelen);
> > +		error = xfs_attr_defer_add(&args);
> > +		if (error)
> > +			goto out_defer_cancel;
> > +	}
> 
> with:
> 
> 	if (parent) {
> 		error = xfs_parent_defer_add(tp, tdp, parent,
> diroffset);
> 		if (error)
> 			goto out_defer_cancel;
> 	}
> 
> and implement it something like:
> 
> int
> xfs_parent_defer_add(
> 	struct xfs_trans	*tp,
> 	struct xfs_inode	*ip,
> 	struct xfs_parent_defer	*parent,
> 	xfs_dir2_dataptr_t	diroffset)
> {
> 	struct xfs_da_args	*args = &parent->args;
> 
> 	xfs_init_parent_name_rec(&parent->rec, ip, diroffset)
> 	args->trans = tp;
> 	args->hashval = xfs_da_hashname(args->name, args->namelen);
> 	return xfs_attr_defer_add(args);
> }
> 
> 
> > +
> >  	/*
> >  	 * If this is a synchronous mount, make sure that the
> >  	 * link transaction goes to disk before returning to
> > @@ -1331,11 +1367,21 @@ xfs_link(
> >  	if (xfs_has_wsync(mp) || xfs_has_dirsync(mp))
> >  		xfs_trans_set_sync(tp);
> >  
> > -	return xfs_trans_commit(tp);
> > +	error = xfs_trans_commit(tp);
> > +	xfs_iunlock(tdp, XFS_ILOCK_EXCL);
> > +	xfs_iunlock(sip, XFS_ILOCK_EXCL);
> 
> with a xfs_parent_free(parent) added here now that we are done with
> the parent update.
> 
> > +	return error;
> >  
> > - error_return:
> > +out_defer_cancel:
> > +	xfs_defer_cancel(tp);
> > +error_return:
> >  	xfs_trans_cancel(tp);
> > - std_return:
> > +	xfs_iunlock(tdp, XFS_ILOCK_EXCL);
> > +	xfs_iunlock(sip, XFS_ILOCK_EXCL);
> > +drop_incompat:
> > +	if (xfs_has_larp(mp))
> > +		xlog_drop_incompat_feat(mp->m_log);
> 
> And this can be replace with  xfs_parent_cancel(mp, parent); that
> drops the log incompat featuer and frees the parent if it is not
> null.

Sure, that sounds reasonable.  Let me punch it up and see how it does
int the tests.

> 
> > +std_return:
> >  	if (error == -ENOSPC && nospace_error)
> >  		error = nospace_error;
> >  	return error;
> > @@ -2819,7 +2865,7 @@ xfs_remove(
> >  	 */
> >  	resblks = XFS_REMOVE_SPACE_RES(mp);
> >  	error = xfs_trans_alloc_dir(dp, &M_RES(mp)->tr_remove, ip,
> > &resblks,
> > -			&tp, &dontcare);
> > +			&tp, &dontcare, XFS_ILOCK_EXCL);
> 
> So you add this flag here so that link and remove can do different
> things in xfs_trans_alloc_dir(), but in the very next patch
> this gets changed to zero, so both callers only pass 0 to the
> function.
> 
> Ideally there should be a patch prior to this one that converts
> the locking and joining of both link and remove to use external
> inode locking in a single patch, similar to the change in the second
> patch that changed the inode locking around xfs_init_new_inode() to
> require manual unlock. Then all the locking mods in this and the
> next patch go away, leaving just the parent pointer mods in this
> patch....
Sure, I can do it that way too.

Thanks for the reviews!
Allison

> 
> Cheers,
> 
> Dave.