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    SubjectRe: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core
    On 07/15/2011 06:23 AM, Peter Zijlstra wrote:
    > 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.
    >

    My fault, that will be fixed in the V2 patch.

    >> + 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!
    >

    because of the kernel read only permission of the page is transparent
    to the follow_page(), the handle_mm_fault() is not to be activated
    in the __get_use_pages(), so the gup(.write=1) could not help to fixup
    the write permission.


    Thanks
    Shan Hai

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



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