On 2023/9/25 23:00, Jarkko Sakkinen wrote:
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 firstname.lastname@example.org Tested-by: Ma Wupeng email@example.com Reviewed-by: Kefeng Wang firstname.lastname@example.org Reviewed-by: Xiaofei Tan email@example.com Reviewed-by: Baolin Wang firstname.lastname@example.org
Did 7f17b4a121d0 actually break something that was not broken before?
If not, this is (afaik) not a bug fix.
It did not. It keeps track of whether memory_failure() work was queued, and makes task_work pending to flush out the queue. But if no work queued for synchronous error due to abnormal branches, it does not do a force kill to current process resulting a hard lockup due to exception loop.
It is fine to me to remove the bug fix tag if you insist on removing it.
Best Regards, Shuai