Re: [PATCH 1/5] x86/mm/pat: introcude cpa_lock() and cpa_unlock()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 28, 2026 at 04:07:44PM +0300, Mike Rapoport (Microsoft) wrote:

>  arch/x86/mm/pat/set_memory.c | 33 +++++++++++++++++++++++++--------
>  1 file changed, 25 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index b1e780a465b5..4c8922695fd3 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -65,8 +65,25 @@ static const int cpa_warn_level = CPA_PROTECT;
>   * Serialize cpa() using cpa_lock so that we don't allow any other cpu, with
>   * stale large tlb entries, to change the page attribute in parallel to some
>   * other cpu splitting a large page entry along with changing the attribute.
> + *
> + * When debug_pagealloc_enabled(), page attributes could be changed in atomic
> + * context that would warrant disabling IRQs. But since debug_pagealloc always
> + * uses 4k pages in the direct map there are no races for splits and collapses
> + * and locking can be just skipped altogether.
>   */
> -static DEFINE_SPINLOCK(cpa_lock);
> +static DEFINE_SPINLOCK(_cpa_lock);
> +
> +static inline void cpa_lock(void)
> +{
> +	if (!debug_pagealloc_enabled())
> +		spin_lock(&_cpa_lock);
> +}
> +
> +static inline void cpa_unlock(void)
> +{
> +	if (!debug_pagealloc_enabled())
> +		spin_unlock(&_cpa_lock);
> +}

If this lives, we should probably stick an assertion in both the split
and alloc cases for holding _cpa_lock. That debug thing seems to rely on
never hitting those, but having that be implicit is asking for pain.