Messages in this thread | | | Date | Tue, 28 Dec 2021 18:26:39 +0800 | From | Kefeng Wang <> | Subject | Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings |
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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@gmail.com> 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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