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    Subject[RFC PATCH 12/26] x86/alternatives: Use __get_unlocked_pte() in text_poke()
    Date
    text_poke() uses get_locked_pte() to map poking_addr. However, this
    introduces a dependency on locking code which precludes using
    text_poke() to modify qspinlock primitives.

    Accesses to this pte (and poking_addr) are protected by text_mutex
    so we can safely switch to __get_unlocked_pte() here. Note that
    we do need to be careful that we do not try to modify the poking_addr
    from multiple contexts simultaneously (ex. INT3 or NMI context.)

    Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
    ---
    arch/x86/kernel/alternative.c | 9 ++++-----
    include/linux/mm.h | 16 ++++++++++++++--
    mm/memory.c | 9 ++++++---
    3 files changed, 24 insertions(+), 10 deletions(-)

    diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
    index 8c79a3dc5e72..0344e49a4ade 100644
    --- a/arch/x86/kernel/alternative.c
    +++ b/arch/x86/kernel/alternative.c
    @@ -812,7 +812,6 @@ static void __text_poke(void *addr, const void *opcode, size_t len)
    temp_mm_state_t prev;
    unsigned long flags;
    pte_t pte, *ptep;
    - spinlock_t *ptl;
    pgprot_t pgprot;

    /*
    @@ -846,10 +845,11 @@ static void __text_poke(void *addr, const void *opcode, size_t len)
    pgprot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);

    /*
    - * The lock is not really needed, but this allows to avoid open-coding.
    + * text_poke() might be used to poke spinlock primitives so do this
    + * unlocked. This does mean that we need to be careful that no other
    + * context (ex. INT3 handler) is simultaneously writing to this pte.
    */
    - ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
    -
    + ptep = __get_unlocked_pte(poking_mm, poking_addr);
    /*
    * This must not fail; preallocated in poking_init().
    */
    @@ -904,7 +904,6 @@ static void __text_poke(void *addr, const void *opcode, size_t len)
    */
    BUG_ON(memcmp(addr, opcode, len));

    - pte_unmap_unlock(ptep, ptl);
    local_irq_restore(flags);
    }

    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 7dd5c4ccbf85..d4a652c2e269 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -1895,8 +1895,20 @@ static inline int pte_devmap(pte_t pte)

    int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);

    -extern pte_t *__get_locked_pte(struct mm_struct *mm, unsigned long addr,
    - spinlock_t **ptl);
    +pte_t *__get_pte(struct mm_struct *mm, unsigned long addr, spinlock_t **ptl);
    +
    +static inline pte_t *__get_unlocked_pte(struct mm_struct *mm,
    + unsigned long addr)
    +{
    + return __get_pte(mm, addr, NULL);
    +}
    +
    +static inline pte_t *__get_locked_pte(struct mm_struct *mm,
    + unsigned long addr, spinlock_t **ptl)
    +{
    + return __get_pte(mm, addr, ptl);
    +}
    +
    static inline pte_t *get_locked_pte(struct mm_struct *mm, unsigned long addr,
    spinlock_t **ptl)
    {
    diff --git a/mm/memory.c b/mm/memory.c
    index 586271f3efc6..7acfe9512084 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -1407,8 +1407,8 @@ void zap_vma_ptes(struct vm_area_struct *vma, unsigned long address,
    }
    EXPORT_SYMBOL_GPL(zap_vma_ptes);

    -pte_t *__get_locked_pte(struct mm_struct *mm, unsigned long addr,
    - spinlock_t **ptl)
    +pte_t *__get_pte(struct mm_struct *mm, unsigned long addr,
    + spinlock_t **ptl)
    {
    pgd_t *pgd;
    p4d_t *p4d;
    @@ -1427,7 +1427,10 @@ pte_t *__get_locked_pte(struct mm_struct *mm, unsigned long addr,
    return NULL;

    VM_BUG_ON(pmd_trans_huge(*pmd));
    - return pte_alloc_map_lock(mm, pmd, addr, ptl);
    + if (likely(ptl))
    + return pte_alloc_map_lock(mm, pmd, addr, ptl);
    + else
    + return pte_alloc_map(mm, pmd, addr);
    }

    /*
    --
    2.20.1
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