On Tue, Mar 07, 2023 at 01:44:05AM +0000, Nadav Amit wrote: > > > > 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. In general. IIRC there're more than one such case in this patch. > 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. Thanks for the knowledge. I would suspect no matter how the output layout of the compilers will be the difference will be small. I prefer it more for readability as you said but not strongly either way. -- Peter Xu
