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    SubjectRe: [PATCH v2 06/35] nds32: MMU fault handling and page table management
    Hi,

    On Mon, Nov 27, 2017 at 08:27:53PM +0800, Greentime Hu wrote:
    > +void do_page_fault(unsigned long entry, unsigned long addr,
    > + unsigned int error_code, struct pt_regs *regs)
    > +{

    > + /*
    > + * As per x86, we may deadlock here. However, since the kernel only
    > + * validly references user space from well defined areas of the code,
    > + * we can bug out early if this is from code which shouldn't.
    > + */
    > + if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
    > + if (!user_mode(regs) &&
    > + !search_exception_tables(instruction_pointer(regs)))
    > + goto no_context;
    > +retry:
    > + down_read(&mm->mmap_sem);
    > + } else {
    > + /*
    > + * The above down_read_trylock() might have succeeded in which
    > + * case, we'll have missed the might_sleep() from down_read().
    > + */
    > + might_sleep();
    > + if (IS_ENABLED(CONFIG_DEBUG_VM)) {
    > + if (!user_mode(regs) &&
    > + !search_exception_tables(instruction_pointer(regs)))
    > + goto no_context;
    > + }
    > + }

    > + fault = handle_mm_fault(vma, addr, flags);
    > +
    > + /*
    > + * If we need to retry but a fatal signal is pending, handle the
    > + * signal first. We do not need to release the mmap_sem because it
    > + * would already be released in __lock_page_or_retry in mm/filemap.c.
    > + */
    > + if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
    > + return;

    I believe you can get stuck in a livelock here (with an unkillable
    task), if a uaccess primitive tries to access a region protected by a
    userfaultfd. Please see:

    https://lkml.kernel.org/r/1499782590-31366-1-git-send-email-mark.rutland@arm.com

    ... for details and a test case.

    Thanks,
    Mark.

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