Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings

[Dave Hansen <dave.hansen@intel.com>]

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?