On Mon, Oct 5, 2020 at 6:01 PM Alexander Potapenko <glider@xxxxxxxxxx> wrote: > > On Tue, Sep 29, 2020 at 5:06 PM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > > On Tue, Sep 29, 2020 at 04:51:29PM +0200, Marco Elver wrote: > > > On Tue, 29 Sep 2020 at 16:24, Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > [...] > > > > > > > > From other sub-threads it sounds like these addresses are not part of > > > > the linear/direct map. Having kmalloc return addresses outside of the > > > > linear map is going to break anything that relies on virt<->phys > > > > conversions, and is liable to make DMA corrupt memory. There were > > > > problems of that sort with VMAP_STACK, and this is why kvmalloc() is > > > > separate from kmalloc(). > > > > > > > > Have you tested with CONFIG_DEBUG_VIRTUAL? I'd expect that to scream. > > > > > > > > I strongly suspect this isn't going to be safe unless you always use an > > > > in-place carevout from the linear map (which could be the linear alias > > > > of a static carevout). > > > > > > That's an excellent point, thank you! Indeed, on arm64, a version with > > > naive static-pool screams with CONFIG_DEBUG_VIRTUAL. > > > > > > We'll try to put together an arm64 version using a carveout as you suggest. > > > > Great, thanks! > > > > Just to be clear, the concerns for DMA and virt<->phys conversions also > > apply to x86 (the x86 virt<->phys conversion behaviour is more forgiving > > in the common case, but still has cases that can go wrong). > > To clarify, shouldn't kmalloc/kmem_cache allocations used with DMA be > allocated with explicit GFP_DMA? > If so, how practical would it be to just skip such allocations in > KFENCE allocator? AFAIK GFP_DMA doesn't really mean "I will use this allocation for DMA"; it means "I will use this allocation for DMA using some ancient hardware (e.g. stuff on the ISA bus?) that only supports 16-bit physical addresses (or maybe different limits on other architectures)". There's also GFP_DMA32, which means the same thing, except with 32-bit physical addresses. You can see in e.g. __dma_direct_alloc_pages() that the GFP_DMA32 and GFP_DMA flags are only used if the hardware can't address the full physical address space supported by the CPU.
