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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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