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Subject: Re: [PATCH] mm,hwpoison: return -EBUSY when page already poisoned
To:     Aili Yao <yaoaili@kingsoft.com>, naoya.horiguchi@nec.com,
        akpm@linux-foundation.org, tony.luck@intel.com, bp@alien8.de,
        tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com
Cc:     x86@kernel.org, inux-edac@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        yangfeng1@kingsoft.com
References: <20210224151619.67c29731@alex-virtual-machine>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <97a2511e-2002-ec25-6902-8fe841922138@redhat.com>
Date:   Wed, 24 Feb 2021 11:10:23 +0100
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On 24.02.21 08:16, Aili Yao wrote:
> When the page is already poisoned, another memory_failure() call in the
> same page now return 0, meaning OK. For nested memory mce handling, this
> behavior may lead real serious problem, Example:
> 
> 1.When LCME is enabled, and there are two processes A && B running on
> different core X && Y separately, which will access one same page, then
> the page corrupted when process A access it, a MCE will be rasied to
> core X and the error process is just underway.
> 
> 2.Then B access the page and trigger another MCE to core Y, it will also
> do error process, it will see TestSetPageHWPoison be true, and 0 is
> returned.
> 
> 3.The kill_me_maybe will check the return:
> 
> 1244 static void kill_me_maybe(struct callback_head *cb)
> 1245 {
> 
> 1254         if (!memory_failure(p->mce_addr >> PAGE_SHIFT, flags) &&
> 1255             !(p->mce_kflags & MCE_IN_KERNEL_COPYIN)) {
> 1256                 set_mce_nospec(p->mce_addr >> PAGE_SHIFT,
> p->mce_whole_page);
> 1257                 sync_core();
> 1258                 return;
> 1259         }
> 
> 1267 }
> 
> 4. The error process for B will end, and may nothing happened if
> kill-early is not set, We may let the wrong data go into effect.
> 
> For other cases which care the return value of memory_failure() should
> check why they want to process a memory error which have already been
> processed. This behavior seems reasonable.
> 
> In kill_me_maybe, log the fact about the memory may not recovered, and
> we will kill the related process.
>

Is -EBUSY then the right return value?

I'd expect if it's already poisoned that we would get something like 
EHWPOISON.

Does this affect existing user space interfaces (especially, via madvise?)?

-- 
Thanks,

David / dhildenb