| Message ID | 1588907271-11920-2-git-send-email-anshuman.khandual@arm.com (mailing list archive) |
|---|---|
| State | Mainlined |
| Commit | be51e3fde5c10f269c065fdca2caf28e4708e21e |
| Headers | show |
| Series | mm/hugetlb: Add some new generic fallbacks | expand |
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@arm.com/ > > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Will Deacon <will@kernel.org> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-mm@kvack.org > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> > --- > 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. Will was nervous about dropping this from arm64. I'm just a little nervous about adding it to other architectures.
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@arm.com/ >> >> Cc: Catalin Marinas <catalin.marinas@arm.com> >> Cc: Will Deacon <will@kernel.org> >> Cc: Andrew Morton <akpm@linux-foundation.org> >> Cc: linux-arm-kernel@lists.infradead.org >> Cc: linux-mm@kvack.org >> Cc: linux-kernel@vger.kernel.org >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> >> --- >> 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().
On 5/10/20 9:02 PM, Anshuman Khandual wrote: > On 05/09/2020 03:39 AM, Mike Kravetz wrote: >> On 5/7/20 8:07 PM, Anshuman Khandual wrote: >> 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(). I have no specific concerns or objection. After thinking about it some more, I do not see how adding the READ_ONCE could cause any issues. Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
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
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@arm.com/ Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: linux-arm-kernel@lists.infradead.org Cc: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> --- arch/arm64/include/asm/hugetlb.h | 6 ------ include/asm-generic/hugetlb.h | 2 +- 2 files changed, 1 insertion(+), 7 deletions(-)