On 05/05/2026 11:59, Sumit Garg wrote:
> 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 <[email protected]>
>> ---
>> arch/arm/cpu/armv8/cache_v8.c | 16 ++++++++++++++++
>> 1 file changed, 16 insertions(+)
>
> Nice optimization, FWIW:
>
> Acked-by: Sumit Garg <[email protected]>
> Tested-by: Sumit Garg <[email protected]>
Could you drop this on the v2 instead? Sorry for the confusion this
version had a silly bug.
>
> -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
>>
--
// Casey (she/her)
