[PATCH v1 4/4] mm/hwpoison: Fix incorrect "not recovered" report for recovered clean pages

[Shuai Xue <xueshuai@linux.alibaba.com>]

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.

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. However, for clean pages, the
TTU_HWPOISON flag is cleared, leaving the PTE unchanged and not converted
to a hwpoison entry. Consequently, for an unmapped dirty page, the PTE is
marked as a hwpoison entry allowing kill_accessing_process() to:

- call walk_page_range() and return 1
- 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.

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 -EHWPOISON if no hwpoison PTE entry is found, preventing
an unnecessary SIGBUS.

Fixes: 046545a661af ("mm/hwpoison: fix error page recovered but reported "not recovered"")
Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
---
 mm/memory-failure.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 995a15eb67e2..f9a6b136a6f0 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -883,10 +883,9 @@ static int kill_accessing_process(struct task_struct *p, unsigned long pfn,
 			      (void *)&priv);
 	if (ret == 1 && priv.tk.addr)
 		kill_proc(&priv.tk, pfn, flags);
-	else
-		ret = 0;
 	mmap_read_unlock(p->mm);
-	return ret > 0 ? -EHWPOISON : -EFAULT;
+
+	return ret >= 0 ? -EHWPOISON : -EFAULT;
 }
 
 /*
-- 
2.39.3