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    SubjectRe: [RESEND PATCH v8 2/2] ACPI: APEI: handle synchronous exceptions in task work
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    On Tue Sep 19, 2023 at 5:21 AM EEST, Shuai Xue wrote:
    > Hardware errors could be signaled by synchronous interrupt, e.g. when an
    > error is detected by a background scrubber, or signaled by synchronous
    > exception, e.g. when an uncorrected error is consumed. Both synchronous and
    > asynchronous error are queued and handled by a dedicated kthread in
    > workqueue.
    >
    > commit 7f17b4a121d0 ("ACPI: APEI: Kick the memory_failure() queue for
    > synchronous errors") keep track of whether memory_failure() work was
    > queued, and make task_work pending to flush out the workqueue so that the
    > work for synchronous error is processed before returning to user-space.
    > The trick ensures that the corrupted page is unmapped and poisoned. And
    > after returning to user-space, the task starts at current instruction which
    > triggering a page fault in which kernel will send SIGBUS to current process
    > due to VM_FAULT_HWPOISON.
    >
    > However, the memory failure recovery for hwpoison-aware mechanisms does not
    > work as expected. For example, hwpoison-aware user-space processes like
    > QEMU register their customized SIGBUS handler and enable early kill mode by
    > seting PF_MCE_EARLY at initialization. Then the kernel will directy notify
    > the process by sending a SIGBUS signal in memory failure with wrong
    > si_code: the actual user-space process accessing the corrupt memory
    > location, but its memory failure work is handled in a kthread context, so
    > it will send SIGBUS with BUS_MCEERR_AO si_code to the actual user-space
    > process instead of BUS_MCEERR_AR in kill_proc().
    >
    > To this end, separate synchronous and asynchronous error handling into
    > different paths like X86 platform does:
    >
    > - valid synchronous errors: queue a task_work to synchronously send SIGBUS
    > before ret_to_user.
    > - valid asynchronous errors: queue a work into workqueue to asynchronously
    > handle memory failure.
    > - abnormal branches such as invalid PA, unexpected severity, no memory
    > failure config support, invalid GUID section, OOM, etc.
    >
    > Then for valid synchronous errors, the current context in memory failure is
    > exactly belongs to the task consuming poison data and it will send SIBBUS
    > with proper si_code.
    >
    > Fixes: 7f17b4a121d0 ("ACPI: APEI: Kick the memory_failure() queue for synchronous errors")
    > Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
    > Tested-by: Ma Wupeng <mawupeng1@huawei.com>
    > Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
    > Reviewed-by: Xiaofei Tan <tanxiaofei@huawei.com>
    > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

    Did 7f17b4a121d0 actually break something that was not broken before?

    If not, this is (afaik) not a bug fix.

    BR, Jarkko

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