Re: [PATCH v3 tip/perf/core 2/4] mm: switch to 64-bit mm_lock_seq/vm_lock_seq on 64-bit architectures

[Peter Zijlstra]

On Thu, Oct 10, 2024 at 01:56:42PM -0700, Andrii Nakryiko wrote:
> To increase mm->mm_lock_seq robustness, switch it from int to long, so
> that it's a 64-bit counter on 64-bit systems and we can stop worrying
> about it wrapping around in just ~4 billion iterations. Same goes for
> VMA's matching vm_lock_seq, which is derived from mm_lock_seq.
> 
> I didn't use __u64 outright to keep 32-bit architectures unaffected, but
> if it seems important enough, I have nothing against using __u64.

(__uXX are the uapi types)

> 
> Suggested-by: Jann Horn <jannh@xxxxxxxxxx>

Jann, do you see problems with the normal seqcount being unsigned (int)?
I suppose especially for preemptible seqcounts it might already be
entirely feasible to wrap them?

Doing u64 is tricky but not impossible, it would require something like
we do for GUP_GET_PXX_LOW_HIGH. OTOH, I don't think we really care about
32bit enough to bother.