On 12/27/21 6:59 AM, Kefeng Wang wrote:
> This patch select HAVE_ARCH_HUGE_VMALLOC to let X86_64 and X86_PAE
> support huge vmalloc mappings.
In general, this seems interesting and the diff is simple. But, I don't
see _any_ x86-specific data. I think the bare minimum here would be a
few kernel compiles and some 'perf stat' data for some TLB events.
> diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
> index 95fa745e310a..6bf5cb7d876a 100644
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -75,8 +75,8 @@ void *module_alloc(unsigned long size)
>
> p = __vmalloc_node_range(size, MODULE_ALIGN,
> MODULES_VADDR + get_module_load_offset(),
> - MODULES_END, gfp_mask,
> - PAGE_KERNEL, VM_DEFER_KMEMLEAK, NUMA_NO_NODE,
> + MODULES_END, gfp_mask, PAGE_KERNEL,
> + VM_DEFER_KMEMLEAK | VM_NO_HUGE_VMAP, NUMA_NO_NODE,
> __builtin_return_address(0));
> if (p && (kasan_module_alloc(p, size, gfp_mask) < 0)) {
> vfree(p);
To figure out what's going on in this hunk, I had to look at the cover
letter (which I wasn't cc'd on). That's not great and it means that
somebody who stumbles upon this in the code is going to have a really
hard time figuring out what is going on. Cover letters don't make it
into git history.
This desperately needs a comment and some changelog material in *this*
patch.
But, even the description from the cover letter is sparse:
> There are some disadvantages about this feature[2], one of the main
> concerns is the possible memory fragmentation/waste in some scenarios,
> also archs must ensure that any arch specific vmalloc allocations that
> require PAGE_SIZE mappings(eg, module alloc with STRICT_MODULE_RWX)
> use the VM_NO_HUGE_VMAP flag to inhibit larger mappings.
That just says that x86 *needs* PAGE_SIZE allocations. But, what
happens if VM_NO_HUGE_VMAP is not passed (like it was in v1)? Will the
subsequent permission changes just fragment the 2M mapping?
