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    Subject[PATCH 5/6] x86, mce: handle "action required" errors (unjumbled version)
    All non-urgent actions (reporting low severity errors and handling
    "action-optional" errors) are now handled by a work queue. This
    means that TIF_MCE_NOTIFY can be used to block execution for a
    thread experiencing an "action-required" fault until we get all
    cpus out of the machine check handler (and the thread that hit
    the fault into mce_notify_process().

    We use the new mce_{save,find,clear}_info() API to get information
    from do_machine_check() to mce_notify_process(), and then use the
    newly improved memory_failure(..., MF_ACTION_REQUIRED) to handle
    the error (possibly signalling the process).

    Update some comments to make the new code flows clearer.

    Signed-off-by: Tony Luck <tony.luck@intel.com>
    ---

    [Version posted earlier had the diff included in the commit comment,
    and then the real copy of the diff - making it hard to read. Sorry.]

    arch/x86/kernel/cpu/mcheck/mce.c | 71 ++++++++++++++++++++++++--------------
    1 files changed, 45 insertions(+), 26 deletions(-)

    diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
    index 7d7303a..dfd20a6 100644
    --- a/arch/x86/kernel/cpu/mcheck/mce.c
    +++ b/arch/x86/kernel/cpu/mcheck/mce.c
    @@ -982,7 +982,9 @@ void do_machine_check(struct pt_regs *regs, long error_code)
    barrier();

    /*
    - * When no restart IP must always kill or panic.
    + * When no restart IP might need to kill or panic.
    + * Assume the worst for now, but if we find the
    + * severity is MCE_AR_SEVERITY we have other options.
    */
    if (!(m.mcgstatus & MCG_STATUS_RIPV))
    kill_it = 1;
    @@ -1036,12 +1038,6 @@ void do_machine_check(struct pt_regs *regs, long error_code)
    continue;
    }

    - /*
    - * Kill on action required.
    - */
    - if (severity == MCE_AR_SEVERITY)
    - kill_it = 1;
    -
    mce_read_aux(&m, i);

    /*
    @@ -1062,6 +1058,8 @@ void do_machine_check(struct pt_regs *regs, long error_code)
    }
    }

    + m = *final;
    +
    if (!no_way_out)
    mce_clear_state(toclear);

    @@ -1080,7 +1078,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
    * support MCE broadcasting or it has been disabled.
    */
    if (no_way_out && tolerant < 3)
    - mce_panic("Fatal machine check on current CPU", final, msg);
    + mce_panic("Fatal machine check on current CPU", &m, msg);

    /*
    * If the error seems to be unrecoverable, something should be
    @@ -1089,11 +1087,13 @@ void do_machine_check(struct pt_regs *regs, long error_code)
    * high, don't try to do anything at all.
    */

    - if (kill_it && tolerant < 3)
    + if (worst != MCE_AR_SEVERITY && kill_it && tolerant < 3)
    force_sig(SIGBUS, current);

    - /* notify userspace ASAP */
    - set_thread_flag(TIF_MCE_NOTIFY);
    + if (worst == MCE_AR_SEVERITY) {
    + mce_save_info(m.addr);
    + set_thread_flag(TIF_MCE_NOTIFY);
    + }

    if (worst > 0)
    mce_report_event(regs);
    @@ -1107,6 +1107,8 @@ EXPORT_SYMBOL_GPL(do_machine_check);
    #ifndef CONFIG_MEMORY_FAILURE
    int memory_failure(unsigned long pfn, int vector, int flags)
    {
    + if (flags & MF_ACTION_REQUIRED)
    + return -ENXIO; /* panic? */
    printk(KERN_ERR "Uncorrected memory error in page 0x%lx ignored\n"
    "Rebuild kernel with CONFIG_MEMORY_FAILURE=y for smarter handling\n", pfn);

    @@ -1115,27 +1117,46 @@ int memory_failure(unsigned long pfn, int vector, int flags)
    #endif

    /*
    - * Called after mce notification in process context. This code
    - * is allowed to sleep. Call the high level VM handler to process
    - * any corrupted pages.
    - * Assume that the work queue code only calls this one at a time
    - * per CPU.
    - * Note we don't disable preemption, so this code might run on the wrong
    - * CPU. In this case the event is picked up by the scheduled work queue.
    - * This is merely a fast path to expedite processing in some common
    - * cases.
    + * Called in process context that interrupted by MCE and marked with
    + * TIF_MCE_NOTFY, just before returning to errorneous userland.
    + * This code is allowed to sleep.
    + * Attempt possible recovery such as calling the high level VM handler to
    + * process any corrupted pages, and kill/signal current process if required.
    + * Action required errors are handled here.
    */
    void mce_notify_process(void)
    {
    unsigned long pfn;
    - mce_notify_irq();
    - while (mce_ring_get(&pfn))
    - memory_failure(pfn, MCE_VECTOR, 0);
    + struct mce_info *mi = mce_find_info();
    +
    + if (!mi)
    + mce_panic("Lost address", NULL, NULL);
    + pfn = mi->paddr >> PAGE_SHIFT;
    +
    + clear_thread_flag(TIF_MCE_NOTIFY);
    +
    + pr_err("Uncorrected hardware memory error in user-access at %llx",
    + mi->paddr);
    + if (memory_failure(pfn, MCE_VECTOR, MF_ACTION_REQUIRED) < 0) {
    + pr_err("Memory error not recovered");
    + force_sig(SIGBUS, current);
    + } else {
    + pr_err("Memory error recovered");
    + }
    + mce_clear_info(mi);
    }

    +/*
    + * Action optional processing happens here (picking up
    + * from the list of faulting pages that do_machine_check()
    + * placed into the "ring").
    + */
    static void mce_process_work(struct work_struct *dummy)
    {
    - mce_notify_process();
    + unsigned long pfn;
    +
    + while (mce_ring_get(&pfn))
    + memory_failure(pfn, MCE_VECTOR, 0);
    }

    #ifdef CONFIG_X86_MCE_INTEL
    @@ -1225,8 +1246,6 @@ int mce_notify_irq(void)
    /* Not more than two messages every minute */
    static DEFINE_RATELIMIT_STATE(ratelimit, 60*HZ, 2);

    - clear_thread_flag(TIF_MCE_NOTIFY);
    -
    if (test_and_clear_bit(0, &mce_need_notify)) {
    /* wake processes polling /dev/mcelog */
    wake_up_interruptible(&mce_chrdev_wait);
    --
    1.7.3.1


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