On 05/09/2020 03:39 AM, Mike Kravetz wrote: > On 5/7/20 8:07 PM, Anshuman Khandual wrote: >> Platform specific huge_ptep_get() is required only when fetching the huge >> PTE involves more than just dereferencing the page table pointer. This is >> not the case on arm64 platform. Hence huge_ptep_pte() can be dropped along >> with it's __HAVE_ARCH_HUGE_PTEP_GET subscription. Before that, it updates >> the generic huge_ptep_get() with READ_ONCE() which will prevent known page >> table issues with THP on arm64. >> >> https://lore.kernel.org/r/1506527369-19535-1-git-send-email-will.deacon@xxxxxxx/ >> >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> >> Cc: Will Deacon <will@xxxxxxxxxx> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx >> Cc: linux-mm@xxxxxxxxx >> Cc: linux-kernel@xxxxxxxxxxxxxxx >> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx> >> --- >> arch/arm64/include/asm/hugetlb.h | 6 ------ >> include/asm-generic/hugetlb.h | 2 +- >> 2 files changed, 1 insertion(+), 7 deletions(-) >> >> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h >> index 2eb6c234d594..b88878ddc88b 100644 >> --- a/arch/arm64/include/asm/hugetlb.h >> +++ b/arch/arm64/include/asm/hugetlb.h >> @@ -17,12 +17,6 @@ >> extern bool arch_hugetlb_migration_supported(struct hstate *h); >> #endif >> >> -#define __HAVE_ARCH_HUGE_PTEP_GET >> -static inline pte_t huge_ptep_get(pte_t *ptep) >> -{ >> - return READ_ONCE(*ptep); >> -} >> - >> static inline int is_hugepage_only_range(struct mm_struct *mm, >> unsigned long addr, unsigned long len) >> { >> diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h >> index 822f433ac95c..40f85decc2ee 100644 >> --- a/include/asm-generic/hugetlb.h >> +++ b/include/asm-generic/hugetlb.h >> @@ -122,7 +122,7 @@ static inline int huge_ptep_set_access_flags(struct vm_area_struct *vma, >> #ifndef __HAVE_ARCH_HUGE_PTEP_GET >> static inline pte_t huge_ptep_get(pte_t *ptep) >> { >> - return *ptep; >> + return READ_ONCE(*ptep); >> } >> #endif > > I know you made this change in response to Will's comment. And, since > changes were made to consistently use READ_ONCE in arm64 code, it makes > sense for that architecture. > > However, with this change to generic code, you introduce READ_ONCE to > other architectures where it was not used before. Could this possibly > introduce inconsistencies in their use of READ_ONCE? To be honest, I > am not very good at identifying any possible issues this could cause. > However, it does seem possible. Could you please give some more details. Is there any particular problem which might be caused by this new READ_ONCE() here, that you you are concerned about. READ_ONCE() is already getting used in multiple places in core MM which can not be configured out (like mm/gup.c). It is getting used in core HugeTLB (mm/hugetlb.c) as well. AFAICS, there is no standard for using READ_ONCE() while walking page tables entries. We have examples in core MM for both ways. > > Will was nervous about dropping this from arm64. I'm just a little nervous > about adding it to other architectures. > AFAICS, __HAVE_ARCH_HUGE_PTEP_GET should be used on a platform only when a HugeTLB entry could not constructed by dereferencing a page table entry as in the case with ARM (32 bit). Using READ_ONCE() while dereferencing is really not a special case that will need __HAVE_ARCH_HUGE_PTEP_GET. Moving READ_ONCE() into generic definition solves the problem while also taking care of a known problem on arm64. IMHO, it seems like the right thing to do unless there is another problem that pops up some where else because of READ_ONCE().
