Le 29/12/2021 à 12:01, Kefeng Wang a écrit :
>
> 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.
I see no reason to have this flag by default.
Only ARM should have it if necessary, with a comment explaining why just
like powerpc.
And maybe the flag should be there only when STRICT_MODULE_RWX is selected.
>
>> }
>>
>> 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.
>> .
