Re: [PATCH v4 2/3] mm/hwpoison: Do not send SIGBUS to processes with recovered clean pages

[Miaohe Lin <linmiaohe@huawei.com>]

On 2025/3/7 13:44, Shuai Xue wrote:
> When an uncorrected memory error is consumed there is a race between the
> CMCI from the memory controller reporting an uncorrected error with a UCNA
> signature, and the core reporting and SRAR signature machine check when the
> data is about to be consumed.
> 
> - Background: why *UN*corrected errors tied to *C*MCI in Intel platform [1]
> 
> Prior to Icelake memory controllers reported patrol scrub events that
> detected a previously unseen uncorrected error in memory by signaling a
> broadcast machine check with an SRAO (Software Recoverable Action Optional)
> signature in the machine check bank. This was overkill because it's not an
> urgent problem that no core is on the verge of consuming that bad data.
> It's also found that multi SRAO UCE may cause nested MCE interrupts and
> finally become an IERR.
> 
> Hence, Intel downgrades the machine check bank signature of patrol
> scrub from SRAO to UCNA (Uncorrected, No Action required), and signal
> changed to #CMCI. Just to add to the confusion, Linux does take an action
> (in uc_decode_notifier()) to try to offline the page despite the UC*NA*
> signature name.
> 
> - Background: why #CMCI and #MCE race when poison is consuming in Intel platform [1]
> 
> Having decided that CMCI/UCNA is the best action for patrol scrub errors,
> the memory controller uses it for reads too. But the memory controller is
> executing asynchronously from the core, and can't tell the difference
> between a "real" read and a speculative read. So it will do CMCI/UCNA if an
> error is found in any read.
> 
> Thus:
> 
> 1) Core is clever and thinks address A is needed soon, issues a speculative read.
> 2) Core finds it is going to use address A soon after sending the read request
> 3) The CMCI from the memory controller is in a race with MCE from the core
>    that will soon try to retire the load from address A.
> 
> Quite often (because speculation has got better) the CMCI from the memory
> controller is delivered before the core is committed to the instruction
> reading address A, so the interrupt is taken, and Linux offlines the page
> (marking it as poison).
> 
> - Why user process is killed for instr case
> 
> Commit 046545a661af ("mm/hwpoison: fix error page recovered but reported
> "not recovered"") tries to fix noise message "Memory error not recovered"
> and skips duplicate SIGBUSs due to the race. But it also introduced a bug
> that kill_accessing_process() return -EHWPOISON for instr case, as result,
> kill_me_maybe() send a SIGBUS to user process.
> 
> If the CMCI wins that race, the page is marked poisoned when
> uc_decode_notifier() calls memory_failure(). For dirty pages,
> memory_failure() invokes try_to_unmap() with the TTU_HWPOISON flag,
> converting the PTE to a hwpoison entry. As a result,
> kill_accessing_process():
> 
> - call walk_page_range() and return 1 regardless of whether
>   try_to_unmap() succeeds or fails,
> - call kill_proc() to make sure a SIGBUS is sent
> - return -EHWPOISON to indicate that SIGBUS is already sent to the
>   process and kill_me_maybe() doesn't have to send it again.
> 
> However, for clean pages, the TTU_HWPOISON flag is cleared, leaving the
> PTE unchanged and not converted to a hwpoison entry. Conversely, for
> clean pages where PTE entries are not marked as hwpoison,
> kill_accessing_process() returns -EFAULT, causing kill_me_maybe() to
> send a SIGBUS.
> 
> Console log looks like this:
> 
>     Memory failure: 0x827ca68: corrupted page was clean: dropped without side effects
>     Memory failure: 0x827ca68: recovery action for clean LRU page: Recovered
>     Memory failure: 0x827ca68: already hardware poisoned
>     mce: Memory error not recovered
> 
> To fix it, return 0 for "corrupted page was clean", preventing an
> unnecessary SIGBUS to user process.
> 
> [1] https://lore.kernel.org/lkml/20250217063335.22257-1-xueshuai@linux.alibaba.com/T/#mba94f1305b3009dd340ce4114d3221fe810d1871
> Fixes: 046545a661af ("mm/hwpoison: fix error page recovered but reported "not recovered"")
> Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
> Cc: stable@vger.kernel.org

Thanks for your detailed commit log. This patch looks good to me.

Acked-by: Miaohe Lin <linmiaohe@huawei.com>

Thanks.
.