Re: [PATCH] mm: Inline vma_needs_copy

[David Hildenbrand <david@redhat.com>]

On 18.06.25 11:39, Vlastimil Babka wrote:
> On 6/18/25 3:42 AM, Yunshui Jiang wrote:
>> From: jiangyunshui <jiangyunshui@kylinos.cn>
>>
>> Since commit bcd51a3c679d ("hugetlb: lazy page table copies
>> in fork()"), the logic about judging whether to copy
>> page table inside func copy_page_range has been extracted
>> into a separate func vma_needs_copy. While this change
>> improves code readability, it also incurs more function call
>> overhead, especially where fork() were frequently called.
>>
>> Inline func vma_needs_copy to optimize the copy_page_range
>> performance. Given that func vma_needs_copy is only called
>> by copy_page_range, inlining it would not cause unacceptable
>> code bloat.
> 
> I'm surprised the compiler doesn't inline it already, if there's a
> single caller. In fact, mine (gcc-14.3 on x86) already does.

Same here

	gcc (GCC) 15.1.1 20250521 (Red Hat 15.1.1-2)
> 
> So I wonder to which extent should we force override wrong compiler
> heuristics?

I think only where we (a) really know better (b) it's crucial for 
performance and (c) modern compilers don't do the right thing.

> Maybe just inline instead of __always_inline would be OK? Is
> that enough of a hint for your compiler?

I would do the same thing (inline as a hint), but some people apparently 
recently argued that newer compiler mostly ignore it either way. At 
least that's what I recall.

-- 
Cheers,

David / dhildenb