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

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Date:   Mon, 27 Dec 2021 07:56:14 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings

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?