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Date:   Sat, 22 May 2021 15:09:00 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc:     linux-mm@kvack.org, Tony Luck <tony.luck@intel.com>,
        Aili Yao <yaoaili@kingsoft.com>,
        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: Re: [PATCH v5 1/3] mm/memory-failure: Use a mutex to avoid
 memory_failure() races
Message-Id: <20210522150900.39d6832a03c5f772911c5b6d@linux-foundation.org>
In-Reply-To: <20210521030156.2612074-2-nao.horiguchi@gmail.com>
References: <20210521030156.2612074-1-nao.horiguchi@gmail.com>
        <20210521030156.2612074-2-nao.horiguchi@gmail.com>
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On Fri, 21 May 2021 12:01:54 +0900 Naoya Horiguchi <nao.horiguchi@gmail.com> wrote:

> There can be races when multiple CPUs consume poison from the same
> page. The first into memory_failure() atomically sets the HWPoison
> page flag and begins hunting for tasks that map this page. Eventually
> it invalidates those mappings and may send a SIGBUS to the affected
> tasks.
> 
> But while all that work is going on, other CPUs see a "success"
> return code from memory_failure() and so they believe the error
> has been handled and continue executing.
> 
> Fix by wrapping most of the internal parts of memory_failure() in
> a mutex.

We can reduce the scope of that mutex, which helps readability at least.

--- a/mm/memory-failure.c~mm-memory-failure-use-a-mutex-to-avoid-memory_failure-races-fix
+++ a/mm/memory-failure.c
@@ -1397,8 +1397,6 @@ out:
 	return rc;
 }
 
-static DEFINE_MUTEX(mf_mutex);
-
 /**
  * memory_failure - Handle memory failure of a page.
  * @pfn: Page Number of the corrupted page
@@ -1425,6 +1423,7 @@ int memory_failure(unsigned long pfn, in
 	int res = 0;
 	unsigned long page_flags;
 	bool retry = true;
+	static DEFINE_MUTEX(mf_mutex);
 
 	if (!sysctl_memory_failure_recovery)
 		panic("Memory failure on page %lx", pfn);
_