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

[Kefeng Wang]

On 2021/12/29 0:14, Dave Hansen wrote:
> On 12/28/21 2:26 AM, Kefeng Wang wrote:
>>>> 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.
> This all sounds very fragile to me.  Every time a new architecture would
> get added for huge vmalloc() support, the developer needs to know to go
> find that architecture's module_alloc() and add this flag.  They next
> guy is going to forget, just like you did.
>
> Considering that this is not a hot path, a weak function would be a nice
> choice:
>
> /* vmalloc() flags used for all module allocations. */
> unsigned long __weak arch_module_vm_flags()
> {
> 	/*
> 	 * Modules use a single, large vmalloc().  Different
> 	 * permissions are applied later and will fragment
> 	 * huge mappings.  Avoid using huge pages for modules.
> 	 */
> 	return VM_NO_HUGE_VMAP;

For x86, it only fragment, but for arm64, due to apply_to_page_range() in

set_memory_*, without this flag maybe crash. Whatever, we need this

flag for module.

> }
>
> Stick that in some the common module code, next to:
>
>> void * __weak module_alloc(unsigned long size)
>> {
>>          return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
> ...
>
> Then, put arch_module_vm_flags() in *all* of the module_alloc()
> implementations, including the generic one.  That way (even with a new
> architecture) whoever copies-and-pastes their module_alloc()
> implementation is likely to get it right.  The next guy who just does a
> "select HAVE_ARCH_HUGE_VMALLOC" will hopefully just work.

OK， Let me check the VM_FLUSH_RESET_PERMS and try about this way.

Thanks.

>
> VM_FLUSH_RESET_PERMS could probably be dealt with in the same way.
> .