Re: [PATCH] mm: don't allow empty relative nodemask in mpol_relative_nodemask()

[Gregory Price]

On Mon, Jun 01, 2026 at 04:32:25PM +0200, David Hildenbrand (Arm) wrote:
> >>
> >> Thank you for taking a shot at fixing the bug report, please let me know what
> >> you think! Have a great day : -)
> > 
> > Hi Joshua.
> > 
> > Indeed, quick and dirty shot.
> > 
> > The problem is that nodes_fold() can't work with the sz == 0. In
> > other words, folding to a 0-bit bitmap is an error. We don't check
> > that on bitmaps level because it's an internal helper, and it's a
> > caller's responsibility to validate the parameters.
> > 
> > nodes_onto(), or more specifically bitmap_onto(), is a different
> > story. In case of empty relmap, the function actually clears all the
> > bits in dst and returns.
> 
> It's very weird that mpol_new_nodemask() (->create() callback) disallows empty
> nodemasks, but mpol_rebind_nodemask() (->rebind() callback) would allow empty
> nodemasks.
> 

Was this actually observed?

mpol_rebind_nodemask() happens when cgroup.cpuset changes, and
cgroup.cpuset cannot be empty.

cpuset only changes with sysfs twiddles or offlining.  In either case,
cpuset *guarantees* that cpuset.mems will never be empty.

So... is this an observed bug or just a statically discovered
"bug" that can't actually be reached?

~Gregory