Re: [PATCH 3/4] xfs: adjust allocation length in xfs_alloc_space_available

[Dave Chinner <david@fromorbit.com>]

On Fri, Dec 16, 2016 at 09:25:44AM +0100, Christoph Hellwig wrote:
> On Fri, Dec 16, 2016 at 11:28:51AM +1100, Dave Chinner wrote:
> > > +	/*
> > > +	 * Clamp maxlen to the amount of free space available for the actual
> > > +	 * extent allocation.
> > > +	 */
> > > +	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
> > > +		args->maxlen = available;
> > > +		ASSERT(args->maxlen > 0);
> > > +	}
> > 
> > I'd love to get rid of all these (int) casts, too...
> 
> The problem here is that we compare 32-bit signed to 32-bit unsigned
> variables.  And given that this is ripe for nasty bugs due to the C
> type promotion rules I'd rather be extra careful.

Yes, I know - it's bitten me several times in the past. The
underlying problem (IMO) is using xfs_extlen_t for things that
aren't physical BMBT extent lengths, which are bound to:

#define MAXEXTLEN       ((xfs_extlen_t)0x001fffff)      /* 21 bits */            

but it's grown to be used for everything that needs to store or
operate on the length of an extent.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com