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    SubjectRe: [RFC PATCH] proc: clear_refs: do not clear reserved pages
    On Sun, 15 Jan 2012, Will Deacon wrote:

    > Hi Hugh,
    >
    > Thanks for the explanation.
    >
    > On Sat, Jan 14, 2012 at 05:36:37PM +0000, Hugh Dickins wrote:
    > > I'm not saying the horrible hack gate_vma mechanism is any safer
    > > than yours (the latest bug in it was fixed all of 13 days ago).
    > > But I am saying that one horrible hack is safer than two.

    Absolutely.

    > Something like what I've got below seems to do the trick, and clear_refs
    > also seems to behave when it's presented with the gate_vma. If Russell is
    > happy with the approach, we can move to the gate_vma in the future.

    I like it much better, although I haven't tested it fully yet.

    However your patch is missing the worst of the current ARM hack I would
    be glad to see go as follows:

    diff --git a/arch/arm/include/asm/mmu_context.h b/arch/arm/include/asm/mmu_context.h
    index 71605d9f8e..876e545297 100644
    --- a/arch/arm/include/asm/mmu_context.h
    +++ b/arch/arm/include/asm/mmu_context.h
    @@ -18,6 +18,7 @@
    #include <asm/cacheflush.h>
    #include <asm/cachetype.h>
    #include <asm/proc-fns.h>
    +#include <asm-generic/mm_hooks.h>

    void __check_kvm_seq(struct mm_struct *mm);

    @@ -133,32 +135,4 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next,
    #define deactivate_mm(tsk,mm) do { } while (0)
    #define activate_mm(prev,next) switch_mm(prev, next, NULL)

    -/*
    - * We are inserting a "fake" vma for the user-accessible vector page so
    - * gdb and friends can get to it through ptrace and /proc/<pid>/mem.
    - * But we also want to remove it before the generic code gets to see it
    - * during process exit or the unmapping of it would cause total havoc.
    - * (the macro is used as remove_vma() is static to mm/mmap.c)
    - */
    -#define arch_exit_mmap(mm) \
    -do { \
    - struct vm_area_struct *high_vma = find_vma(mm, 0xffff0000); \
    - if (high_vma) { \
    - BUG_ON(high_vma->vm_next); /* it should be last */ \
    - if (high_vma->vm_prev) \
    - high_vma->vm_prev->vm_next = NULL; \
    - else \
    - mm->mmap = NULL; \
    - rb_erase(&high_vma->vm_rb, &mm->mm_rb); \
    - mm->mmap_cache = NULL; \
    - mm->map_count--; \
    - remove_vma(high_vma); \
    - } \
    -} while (0)
    -
    -static inline void arch_dup_mmap(struct mm_struct *oldmm,
    - struct mm_struct *mm)
    -{
    -}
    -
    #endif

    Nicolas


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