Messages in this thread |  | | | Date | Sat, 16 Jul 2011 10:50:14 -0400 | | From | Shan Hai <> | | Subject | Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core |
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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. > >> + 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! >
That's right the gup(.write=1) want to do the same thing as the above code snippet, but it failed for the following reason: because the get_user_pages() would not dirty pte for the reason the follow_page() returns not NULL on *present* and *writable* page, the page which holds the lock is present because its a shared page, writable because demand paging set that up so for shared writable page, so the handle_mm_fault() in the __get_user_page() could not be called.
Why the above code could do the same task, because by calling handle_mm_fault() will set pte dirty by [do_annonymous_page(), memory.c] if (vma->vm_flags & VM_WRITE) entry = pte_mkwrite(pte_mkdirty(entry)); Thanks Shan Hai
>> + flush_tlb_page(vma, address); >> + } >> + } >> +#endif >> +} >> + >> #endif /* __KERNEL__ */ >> #endif /* __ASM_POWERPC_TLB_H */
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