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    SubjectRe: [PATCH v3 3/3] arm64: mm: enable per pmd page table lock
    On Mon, Mar 11, 2019 at 12:12:28PM +0000, Mark Rutland wrote:
    > Hi,
    >
    > On Sat, Mar 09, 2019 at 06:19:06PM -0700, Yu Zhao wrote:
    > > Switch from per mm_struct to per pmd page table lock by enabling
    > > ARCH_ENABLE_SPLIT_PMD_PTLOCK. This provides better granularity for
    > > large system.
    > >
    > > I'm not sure if there is contention on mm->page_table_lock. Given
    > > the option comes at no cost (apart from initializing more spin
    > > locks), why not enable it now.
    > >
    > > We only do so when pmd is not folded, so we don't mistakenly call
    > > pgtable_pmd_page_ctor() on pud or p4d in pgd_pgtable_alloc(). (We
    > > check shift against PMD_SHIFT, which is same as PUD_SHIFT when pmd
    > > is folded).
    >
    > Just to check, I take it pgtable_pmd_page_ctor() is now a NOP when the
    > PMD is folded, and this last paragraph is stale?

    Yes, and will remove it.

    > > Signed-off-by: Yu Zhao <yuzhao@google.com>
    > > ---
    > > arch/arm64/Kconfig | 3 +++
    > > arch/arm64/include/asm/pgalloc.h | 12 +++++++++++-
    > > arch/arm64/include/asm/tlb.h | 5 ++++-
    > > 3 files changed, 18 insertions(+), 2 deletions(-)
    > >
    > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
    > > index cfbf307d6dc4..a3b1b789f766 100644
    > > --- a/arch/arm64/Kconfig
    > > +++ b/arch/arm64/Kconfig
    > > @@ -872,6 +872,9 @@ config ARCH_WANT_HUGE_PMD_SHARE
    > > config ARCH_HAS_CACHE_LINE_SIZE
    > > def_bool y
    > >
    > > +config ARCH_ENABLE_SPLIT_PMD_PTLOCK
    > > + def_bool y if PGTABLE_LEVELS > 2
    > > +
    > > config SECCOMP
    > > bool "Enable seccomp to safely compute untrusted bytecode"
    > > ---help---
    > > diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
    > > index 52fa47c73bf0..dabba4b2c61f 100644
    > > --- a/arch/arm64/include/asm/pgalloc.h
    > > +++ b/arch/arm64/include/asm/pgalloc.h
    > > @@ -33,12 +33,22 @@
    > >
    > > static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
    > > {
    > > - return (pmd_t *)__get_free_page(PGALLOC_GFP);
    > > + struct page *page;
    > > +
    > > + page = alloc_page(PGALLOC_GFP);
    > > + if (!page)
    > > + return NULL;
    > > + if (!pgtable_pmd_page_ctor(page)) {
    > > + __free_page(page);
    > > + return NULL;
    > > + }
    > > + return page_address(page);
    > > }
    > >
    > > static inline void pmd_free(struct mm_struct *mm, pmd_t *pmdp)
    > > {
    > > BUG_ON((unsigned long)pmdp & (PAGE_SIZE-1));
    > > + pgtable_pmd_page_dtor(virt_to_page(pmdp));
    > > free_page((unsigned long)pmdp);
    > > }
    >
    > It looks like arm64's existing stage-2 code is inconsistent across
    > alloc/free, and IIUC this change might turn that into a real problem.
    > Currently we allocate all levels of stage-2 table with
    > __get_free_page(), but free them with p?d_free(). We always miss the
    > ctor and always use the dtor.
    >
    > Other than that, this patch looks fine to me, but I'd feel more
    > comfortable if we could first fix the stage-2 code to free those stage-2
    > tables without invoking the dtor.
    >
    > Anshuman, IIRC you had a patch to fix the stage-2 code to not invoke the
    > dtors. If so, could you please post that so that we could take it as a
    > preparatory patch for this series?

    Will do.

    > Thanks,
    > Mark.
    >
    > > diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
    > > index 106fdc951b6e..4e3becfed387 100644
    > > --- a/arch/arm64/include/asm/tlb.h
    > > +++ b/arch/arm64/include/asm/tlb.h
    > > @@ -62,7 +62,10 @@ static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,
    > > static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmdp,
    > > unsigned long addr)
    > > {
    > > - tlb_remove_table(tlb, virt_to_page(pmdp));
    > > + struct page *page = virt_to_page(pmdp);
    > > +
    > > + pgtable_pmd_page_dtor(page);
    > > + tlb_remove_table(tlb, page);
    > > }
    > > #endif
    > >
    > > --
    > > 2.21.0.360.g471c308f928-goog
    > >

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