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    SubjectRe: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core
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    On Fri, 2011-07-15 at 16:07 +0800, Shan Hai wrote:
    > The kernel has no write permission on COW pages by default on e500 core, this
    > will cause endless loop in futex_lock_pi, because futex code assumes the kernel
    > has write permission on COW pages. Grant write permission to the kernel on COW
    > pages when access violation page fault occurs.
    >
    > Signed-off-by: Shan Hai <haishan.bai@gmail.com>
    > ---
    > arch/powerpc/include/asm/futex.h | 11 ++++++++++-
    > arch/powerpc/include/asm/tlb.h | 25 +++++++++++++++++++++++++
    > 2 files changed, 35 insertions(+), 1 deletions(-)
    >
    > diff --git a/arch/powerpc/include/asm/futex.h b/arch/powerpc/include/asm/futex.h
    > index c94e4a3..54c3e74 100644
    > --- a/arch/powerpc/include/asm/futex.h
    > +++ b/arch/powerpc/include/asm/futex.h
    > @@ -8,6 +8,7 @@
    > #include <asm/errno.h>
    > #include <asm/synch.h>
    > #include <asm/asm-compat.h>
    > +#include <asm/tlb.h>
    >
    > #define __futex_atomic_op(insn, ret, oldval, uaddr, oparg) \
    > __asm__ __volatile ( \
    > @@ -113,7 +114,15 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
    > : "cc", "memory");
    >
    > *uval = prev;
    > - return ret;
    > +
    > + /* Futex assumes the kernel has permission to write to
    > + * COW pages, grant the kernel write permission on COW
    > + * pages because it has none by default.
    > + */
    > + if (ret == -EFAULT)
    > + __tlb_fixup_write_permission(current->mm, (unsigned long)uaddr);
    > +
    > + return ret;
    > }
    >
    > #endif /* __KERNEL__ */
    > diff --git a/arch/powerpc/include/asm/tlb.h b/arch/powerpc/include/asm/tlb.h
    > index e2b428b..3863c6a 100644
    > --- a/arch/powerpc/include/asm/tlb.h
    > +++ b/arch/powerpc/include/asm/tlb.h
    > @@ -45,5 +45,30 @@ static inline void __tlb_remove_tlb_entry(struct mmu_gather *tlb, pte_t *ptep,
    > #endif
    > }
    >
    > +/* Grant write permission to the kernel on a page. */
    > +static inline void __tlb_fixup_write_permission(struct mm_struct *mm,
    > + unsigned long address)
    > +{
    > +#if defined(CONFIG_FSL_BOOKE)
    > + /* Grant write permission to the kernel on a page by setting TLB.SW
    > + * bit, the bit setting operation is tricky here, calling
    > + * handle_mm_fault with FAULT_FLAG_WRITE causes _PAGE_DIRTY bit of
    > + * the pte to be set, the _PAGE_DIRTY of the pte is translated into
    > + * TLB.SW on Powerpc e500 core.
    > + */
    > +
    > + struct vm_area_struct *vma;
    > +
    > + vma = find_vma(mm, address);

    Uhm, find_vma() needs mmap_sem, and futex_atomic_cmpxchg_inatomic() is
    most certainly not called with that lock held.

    > + if (likely(vma)) {
    > + /* only fixup present page */
    > + if (follow_page(vma, address, FOLL_WRITE)) {
    > + handle_mm_fault(mm, vma, address, FAULT_FLAG_WRITE);

    So how can this toggle your sw dirty/young tracking, that's pretty much
    what gup(.write=1) does too!

    > + flush_tlb_page(vma, address);
    > + }
    > + }
    > +#endif
    > +}
    > +
    > #endif /* __KERNEL__ */
    > #endif /* __ASM_POWERPC_TLB_H */



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