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

[Christophe Leroy]

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.
>> .