On Thu, 27 Feb 2020 13:41:46 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > > Secondly, huge_pte_offset in mm/hugetlb.c is for ARCH_WANT_GENERAL_HUGETLB, many > > architectures use it, can you make sure there is no issue on all the > > architectures using it with all the version of gcc ? > > > > Thirdly, there are several places use READ_ONCE to access the page table in mm/* > > (e.g. gup_pmd_range), they're also generical for all architectures, and they're > > much more like unnecessary than here, so why there can use but not here? What's > > more, you can read this commit 688272809. > > Apologies for the late reply. > > In commit 20a004e7 the message says that "Whilst there are some scenarios > where this cannot happen ... the overhead of using READ_ONCE/WRITE_ONCE > everywhere is minimal and makes the code an awful lot easier to reason about." > Therefore, a decision was made to ALWAYS use READ_ONCE in the arm64 code > whether or not it was absolutely necessary. Therefore, I do not think > we can assume all the READ_ONCE additions made in 20a004e7 are necessary. > Then the question remains, it it necessary in two statements above? > I do not believe it is necessary. Why? In the statements, > if (!pgd_present(*pgd)) > and > if (!p4d_present(*p4d)) > the variables are only accessed and dereferenced once. I can not imagine > any way in which the compiler could perform multiple accesses of the variable. > > I do believe the READ_ONCE in code accessing the pud and pmd is necessary. > This is because the variables (pud_entry or pmd_entry) are accessed more than > once. And, I could imagine some strange compiler optimization where it would > dereference the pud or pmd pointer more than once. For this same reason > (multiple accesses), I believe the READ_ONCE was added in commit 688272809. > > I am no expert in this area, so corrections/comments appreciated. > > BTW, I still think there may be races present in lookup_address_in_pgd(). > Multiple dereferences of a p4d, pud and pmd are done. Based on Mike's observations I shall drop this patch. If we still believe it is needed, please enhance the changelog, resend and let's take another look.
