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]> -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 >
