Re: [PATCH 2/6] uprobes: introduce is_swbp_at_addr_fast()

[Peter Zijlstra]

On Fri, 2012-04-06 at 00:21 +0200, Oleg Nesterov wrote:
> +int __weak is_swbp_at_addr_fast(unsigned long vaddr)
> +{
> +       uprobe_opcode_t opcode;
> +       int fault;
> +
> +       pagefault_disable();
> +       fault = __copy_from_user_inatomic(&opcode, (void __user*)vaddr,
> +                                                       sizeof(opcode));
> +       pagefault_enable();
> +
> +       if (unlikely(fault)) {
> +               /*
> +                * XXX: read_opcode() lacks FOLL_FORCE, it can fail if
> +                * we race with another thread which does mprotect(NONE)
> +                * after we hit bp.
> +                */
> +               if (read_opcode(current->mm, vaddr, &opcode))
> +                       return -EFAULT;
> +       }
> +
> +       return is_swbp_insn(&opcode);
> +} 

Why bother with the pagefault_disable() and unlikely fault case and not
simply do copy_from_user() and have it deal with the fault if its needed
anyway?
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