linux-kernel - Re: [RESEND PATCH v8 2/2] ACPI: APEI: handle synchronous exceptions in task work

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Date:   Mon, 25 Sep 2023 18:00:52 +0300
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Shuai Xue" <xueshuai@...ux.alibaba.com>, <rafael@...nel.org>,
        <wangkefeng.wang@...wei.com>, <tanxiaofei@...wei.com>,
        <mawupeng1@...wei.com>, <tony.luck@...el.com>,
        <linmiaohe@...wei.com>, <naoya.horiguchi@....com>,
        <james.morse@....com>, <gregkh@...uxfoundation.org>,
        <will@...nel.org>
Cc:     <linux-acpi@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <linux-edac@...r.kernel.org>,
        <acpica-devel@...ts.linuxfoundation.org>, <stable@...r.kernel.org>,
        <x86@...nel.org>, <justin.he@....com>, <ardb@...nel.org>,
        <ying.huang@...el.com>, <ashish.kalra@....com>,
        <baolin.wang@...ux.alibaba.com>, <bp@...en8.de>,
        <tglx@...utronix.de>, <mingo@...hat.com>,
        <dave.hansen@...ux.intel.com>, <lenb@...nel.org>, <hpa@...or.com>,
        <robert.moore@...el.com>, <lvying6@...wei.com>,
        <xiexiuqi@...wei.com>, <zhuo.song@...ux.alibaba.com>
Subject: Re: [RESEND PATCH v8 2/2] ACPI: APEI: handle synchronous exceptions
 in task work

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@...ux.alibaba.com>
> Tested-by: Ma Wupeng <mawupeng1@...wei.com>
> Reviewed-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> Reviewed-by: Xiaofei Tan <tanxiaofei@...wei.com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

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

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

BR, Jarkko