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

    > 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?

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