Re: [PATCH v3 2/5] mm: userfaultfd: don't pass around both mm and vma

[Nadav Amit <namit@vmware.com>]

> On Mar 6, 2023, at 5:03 PM, Peter Xu <peterx@redhat.com> wrote:
> 
> !! External Email
> 
> On Mon, Mar 06, 2023 at 02:50:21PM -0800, Axel Rasmussen wrote:
>> Quite a few userfaultfd functions took both mm and vma pointers as
>> arguments. Since the mm is trivially accessible via vma->vm_mm, there's
>> no reason to pass both; it just needlessly extends the already long
>> argument list.
>> 
>> Get rid of the mm pointer, where possible, to shorten the argument list.
>> 
>> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> 
> One nit below:
> 
>> @@ -6277,7 +6276,7 @@ int hugetlb_mfill_atomic_pte(struct mm_struct *dst_mm,
>>              folio_in_pagecache = true;
>>      }
>> 
>> -     ptl = huge_pte_lock(h, dst_mm, dst_pte);
>> +     ptl = huge_pte_lock(h, dst_vma->vm_mm, dst_pte);
>> 
>>      ret = -EIO;
>>      if (folio_test_hwpoison(folio))
>> @@ -6319,9 +6318,9 @@ int hugetlb_mfill_atomic_pte(struct mm_struct *dst_mm,
>>      if (wp_copy)
>>              _dst_pte = huge_pte_mkuffd_wp(_dst_pte);
>> 
>> -     set_huge_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
>> +     set_huge_pte_at(dst_vma->vm_mm, dst_addr, dst_pte, _dst_pte);
>> 
>> -     hugetlb_count_add(pages_per_huge_page(h), dst_mm);
>> +     hugetlb_count_add(pages_per_huge_page(h), dst_vma->vm_mm);
> 
> When vm_mm referenced multiple times (say, >=3?), let's still cache it in a
> temp var?
> 
> I'm not sure whether compiler is smart enough to already do that with a
> reg, even if so it may slightly improve readability too, imho, by avoiding
> the multiple but same indirection for the reader.

I am not sure if you referred to this code specifically or in general. I once
looked into it, and the compiler is really stupid in this regard and super
conservative when it comes to aliasing. Even if you use “restrict” keyword or
“__pure” or “__const” function attributes, in certain cases (function calls
to other compilation units, or inline assembly - I don’t remember) the
compiler might ignore them. Worse, llvm and gcc are inconsistent.

From code-generated perspective, I did not see a clear cut that benefits
caching over not. From performance perspective the impact is negligible. I
mention all of that because I thought it matters too, but it mostly does
not.

That’s all to say that in most cases, I think that whatever makes the code
more readable should be preferred. I think that you are correct in saying
that “caching” it will make the code more readable, but performance-wise
it is probably meaningless.