Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings
From: Dave Hansen <hidden>
Date: 2021-12-27 16:00:42
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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.
quoted hunk ↗ jump to hunk
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?