Re: [PATCH 1/3] xfs: fix sparse inode limits on runt AG

[Dave Chinner <david@fromorbit.com>]

On Tue, Nov 12, 2024 at 03:15:39PM -0800, Darrick J. Wong wrote:
> On Wed, Nov 13, 2024 at 09:05:14AM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > The runt AG at the end of a filesystem is almost always smaller than
> > the mp->m_sb.sb_agblocks. Unfortunately, when setting the max_agbno
> > limit for the inode chunk allocation, we do not take this into
> > account. This means we can allocate a sparse inode chunk that
> > overlaps beyond the end of an AG. When we go to allocate an inode
> > from that sparse chunk, the irec fails validation because the
> > agbno of the start of the irec is beyond valid limits for the runt
> > AG.
> > 
> > Prevent this from happening by taking into account the size of the
> > runt AG when allocating inode chunks. Also convert the various
> > checks for valid inode chunk agbnos to use xfs_ag_block_count()
> > so that they will also catch such issues in the future.
> > 
> > Fixes: 56d1115c9bc7 ("xfs: allocate sparse inode chunks on full chunk allocation failure")
> 
> Cc: <stable@vger.kernel.org> # v4.2
> 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> >  fs/xfs/libxfs/xfs_ialloc.c | 16 +++++++++-------
> >  1 file changed, 9 insertions(+), 7 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> > index 271855227514..6258527315f2 100644
> > --- a/fs/xfs/libxfs/xfs_ialloc.c
> > +++ b/fs/xfs/libxfs/xfs_ialloc.c
> > @@ -855,7 +855,8 @@ xfs_ialloc_ag_alloc(
> >  		 * the end of the AG.
> >  		 */
> >  		args.min_agbno = args.mp->m_sb.sb_inoalignmt;
> > -		args.max_agbno = round_down(args.mp->m_sb.sb_agblocks,
> > +		args.max_agbno = round_down(xfs_ag_block_count(args.mp,
> > +							pag->pag_agno),
> 
> So if I'm reading this right, the inode cluster allocation checks now
> enforce that we cannot search for free space beyond the actual end of
> the AG, rounded down per inode alignment rules?
> 
> In that case, can this use the cached ag block count:
> 
> 		args.max_agbno = round_down(
> 					pag_group(pag)->xg_block_count,
> 					args.mp->m_sb.sb_inoalignmt);
> 
> rather than recomputing the block count every time?

Eventually, yes. I have another series that pushes the pag further
into these AG size checks all over the code to try to avoid this
entire class of bug in the future (i.e. blindly using mp->m_sb ag
parameters without considering the last AG is a runt).

I am waiting for the group rework to land
before I did any more work on that conversion. However, it is not
yet in the for-next branch, so I'm assuming that it isn't due to be
merged in the upcoming merge window because that's about to open
and none of that code has seen any time in linux-next of fs-next...

I want this fix to land sooner rather than later, so I used
xfs_ag_block_count() to avoid conflicts with pending work as well
as not require me to chase random dev branches to submit what is a
relatively simple bug fix....

-Dave.
-- 
Dave Chinner
david@fromorbit.com