Re: [PATCH] armv8: mmu: support unmapping regions with set_one_region()

[Sumit Garg <sumit.garg@kernel.org>]

On Mon, May 04, 2026 at 07:53:54PM +0200, Casey Connolly wrote:
> Currently set_one_region() implicitly assumes that we want to map a
> region and aggressively splits blocks into tables to do this, but when
> called with PTE_TYPE_FAULT to unmap a currently mapped region it may
> try to unnecessarily split blocks which doesn't make sense if the entire
> block should actually be unmapped. In the end it then has to walk every
> single page and create a bunch of empty tables.
> 
> Introduce a check for this kind of behaviour and optimise with a fast
> path, if we're unmapping a region >= the size of this entry then we can
> just unmap the entire PTE and whatever it contains.
> 
> This fixes some bogus empty tables being left behind when carving out
> reserved memory regions on Qualcomm, and should improve the performance
> of the break-before-make in mmu_change_region_attr().
> 
> Signed-off-by: Casey Connolly <casey.connolly@linaro.org>
> ---
>  arch/arm/cpu/armv8/cache_v8.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)

Nice optimization, FWIW:

Acked-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Tested-by: Sumit Garg <sumit.garg@oss.qualcomm.com>

-Sumit

> 
> diff --git a/arch/arm/cpu/armv8/cache_v8.c b/arch/arm/cpu/armv8/cache_v8.c
> index 39479df7b21f..b1a1fd571341 100644
> --- a/arch/arm/cpu/armv8/cache_v8.c
> +++ b/arch/arm/cpu/armv8/cache_v8.c
> @@ -939,8 +939,20 @@ static u64 set_one_region(u64 start, u64 size, u64 attrs, bool flag, int level)
>  	int levelshift = level2shift(level);
>  	u64 levelsize = 1ULL << levelshift;
>  	u64 *pte = find_pte(start, level);
>  
> +	/*
> +	 * If we're trying to unmap a region then check if it's already unmapped or if it's bigger
> +	 * then the PTE we're looking at right now, in the first case we can do nothing and in the
> +	 * second case we just need to unmap this page/block.
> +	 * Otherwise we will needlessly create new tables until we have traversed every single page
> +	 * in the region.
> +	 */
> +	if (attrs == PTE_TYPE_FAULT && (pte_type(pte) == PTE_TYPE_FAULT || size >= levelsize)) {
> +		*pte &= ~PMD_ATTRMASK;
> +		return levelsize;
> +	}
> +
>  	/* Can we can just modify the current level block PTE? */
>  	if (is_aligned(start, size, levelsize)) {
>  		if (flag) {
>  			*pte &= ~PMD_ATTRMASK;
> @@ -1081,8 +1093,12 @@ void mmu_change_region_attr(phys_addr_t addr, size_t siz, u64 attrs)
>  	flush_dcache_range(gd->arch.tlb_addr,
>  			   gd->arch.tlb_addr + gd->arch.tlb_size);
>  	__asm_invalidate_tlb_all();
>  
> +	/* If we were unmapping a region then we're done! */
> +	if (attrs == PTE_TYPE_FAULT)
> +		return;
> +
>  	mmu_change_region_attr_nobreak(addr, siz, attrs);
>  }
>  
>  int pgprot_set_attrs(phys_addr_t addr, size_t size, enum pgprot_attrs perm)
> -- 
> 2.53.0
>