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

[Kefeng Wang]

On 2021/12/27 23:56, Dave Hansen wrote:
> 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.

When the feature supported on ppc,

commit 8abddd968a303db75e4debe77a3df484164f1f33
Author: Nicholas Piggin <npiggin@xxxxxxxxx>
Date:   Mon May 3 19:17:55 2021 +1000

    powerpc/64s/radix: Enable huge vmalloc mappings

    This reduces TLB misses by nearly 30x on a `git diff` workload on a
    2-node POWER9 (59,800 -> 2,100) and reduces CPU cycles by 0.54%, due
    to vfs hashes being allocated with 2MB pages.

But the data could be different on different machine/arch.

> > 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.
Sorry for that, will add more into arch's patch changelog.
> 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?
> .

Yes, without VM_NO_HUGE_VMAP, it could fragment the 2M mapping.

When module alloc with STRICT_MODULE_RWX on x86, it calls 
__change_page_attr()

from set_memory_ro/rw/nx which will split large page, so there is no 
need to make

module alloc with HUGE_VMALLOC.

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