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From:   Naoya Horiguchi <nao.horiguchi@gmail.com>
To:     linux-mm@kvack.org, Tony Luck <tony.luck@intel.com>,
        Aili Yao <yaoaili@kingsoft.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        David Hildenbrand <david@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Jue Wang <juew@google.com>, linux-kernel@vger.kernel.org
Subject: [PATCH v5 2/3] mm,hwpoison: Return -EHWPOISON to denote that the page has already been poisoned
Date:   Fri, 21 May 2021 12:01:55 +0900
Message-Id: <20210521030156.2612074-3-nao.horiguchi@gmail.com>
In-Reply-To: <20210521030156.2612074-1-nao.horiguchi@gmail.com>
References: <20210521030156.2612074-1-nao.horiguchi@gmail.com>
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From: Aili Yao <yaoaili@kingsoft.com>

When memory_failure() is called with MF_ACTION_REQUIRED on the
page that has already been hwpoisoned, memory_failure() could fail
to send SIGBUS to the affected process, which results in infinite
loop of MCEs.

Currently memory_failure() returns 0 if it's called for already
hwpoisoned page, then the caller, kill_me_maybe(), could return
without sending SIGBUS to current process.  An action required MCE
is raised when the current process accesses to the broken memory,
so no SIGBUS means that the current process continues to run and
access to the error page again soon, so running into MCE loop.

This issue can arise for example in the following scenarios:

  - Two or more threads access to the poisoned page concurrently.
    If local MCE is enabled, MCE handler independently handles the
    MCE events.  So there's a race among MCE events, and the
    second or latter threads fall into the situation in question.

  - If there was a precedent memory error event and memory_failure()
    for the event failed to unmap the error page for some reason,
    the subsequent memory access to the error page triggers the
    MCE loop situation.

To fix the issue, make memory_failure() return an error code when the
error page has already been hwpoisoned.  This allows memory error
handler to control how it sends signals to userspace.  And make sure
that any process touching a hwpoisoned page should get a SIGBUS even
in "already hwpoisoned" path of memory_failure() as is done in page
fault path.

Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
---
ChangeLog v5:
- update patch description.
---
 mm/memory-failure.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git v5.13-rc2/mm/memory-failure.c v5.13-rc2_patched/mm/memory-failure.c
index 0f0b932ccbca..8add7cafad5e 100644
--- v5.13-rc2/mm/memory-failure.c
+++ v5.13-rc2_patched/mm/memory-failure.c
@@ -1247,7 +1247,7 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
 	if (TestSetPageHWPoison(head)) {
 		pr_err("Memory failure: %#lx: already hardware poisoned\n",
 		       pfn);
-		return 0;
+		return -EHWPOISON;
 	}
 
 	num_poisoned_pages_inc();
@@ -1456,6 +1456,7 @@ int memory_failure(unsigned long pfn, int flags)
 	if (TestSetPageHWPoison(p)) {
 		pr_err("Memory failure: %#lx: already hardware poisoned\n",
 			pfn);
+		res = -EHWPOISON;
 		goto unlock_mutex;
 	}
 
-- 
2.25.1