> On Mar 6, 2023, at 5:03 PM, Peter Xu <peterx@xxxxxxxxxx> 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@xxxxxxxxxx> > > Acked-by: Peter Xu <peterx@xxxxxxxxxx> > > 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.
