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

From: Dave Hansen <dave.hansen@intel.com>
To: Kefeng Wang <wangkefeng.wang@huawei.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
	linux-arm-kernel@lists.infradead.org
Cc: Nicholas Piggin <npiggin@gmail.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings
Date: Mon, 27 Dec 2021 07:56:14 -0800	[thread overview]
Message-ID: <70ff58bc-3a92-55c2-2da8-c5877af72e44@intel.com> (raw)
In-Reply-To: <20211227145903.187152-4-wangkefeng.wang@huawei.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?