Re: [PATCH] xfs: ensure st_blocks never goes to zero during COW writes

[Christoph Hellwig <hch@lst.de>]

On Tue, Aug 20, 2024 at 09:44:27PM -0700, Darrick J. Wong wrote:
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> How hard is it to hit this race?a  I guess all you have to do is statx
> in a loop while doing a bunch of cow writeback?

Yes.  That's essential what generic/615 is doing.  Now without always_cow
you'd need to do this on freshly reflinked inodes, which makes the
reproducer a bit harder to create.

> >  	ip->i_nblocks += len;
> > +	ip->i_delayed_blks -= len;
> 
> This proabably ought to have a comment to reference xfs_bmap_defer_add.

Ok.

> >  	bi->bi_owner = *ipp;
> >  	xfs_bmap_update_get_group(mp, bi);
> >  
> > +	/* see __xfs_bmap_add for details */
> 
> xfs_bmap_defer_add?

Yes.