On Tue, Sep 22, 2020 at 11:56:26AM +0200, Marco Elver wrote: > On Mon, 21 Sep 2020 at 19:44, Will Deacon <will@xxxxxxxxxx> wrote: > [...] > > > > > > For ARM64, we would like to solicit feedback on what the best option is > > > > > > to obtain a constant address for __kfence_pool. One option is to declare > > > > > > a memory range in the memory layout to be dedicated to KFENCE (like is > > > > > > done for KASAN), however, it is unclear if this is the best available > > > > > > option. We would like to avoid touching the memory layout. > > > > > > > > > > Sorry for the delay on this. > > > > > > > > NP, thanks for looking! > > > > > > > > > Given that the pool is relatively small (i.e. when compared with our virtual > > > > > address space), dedicating an area of virtual space sounds like it makes > > > > > the most sense here. How early do you need it to be available? > > > > > > > > Yes, having a dedicated address sounds good. > > > > We're inserting kfence_init() into start_kernel() after timekeeping_init(). > > > > So way after mm_init(), if that matters. > > > > > > The question is though, how big should that dedicated area be? > > > Right now KFENCE_NUM_OBJECTS can be up to 16383 (which makes the pool > > > size 64MB), but this number actually comes from the limitation on > > > static objects, so we might want to increase that number on arm64. > > > > What happens on x86 and why would we do something different? > > On x86 we just do `char __kfence_pool[KFENCE_POOL_SIZE] ...;` to > statically allocate the pool. On arm64 this doesn't seem to work > because static memory doesn't have struct pages? Are you using virt_to_page() directly on that statically-allocated __kfence_pool? If so you'll need to use lm_alias() if so, as is done in mm/kasan/init.c. Anything statically allocated is part of the kernel image address range rather than the linear/direct map, and doesn't have a valid virt addr, but its linear map alias does. If you enable CONFIG_DEBUG_VIRTUAL you should get warnings if missing lm_alias() calls. Thanks, Mark.
