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From:   Jue Wang <juew@google.com>
Date:   Thu, 22 Jul 2021 21:16:40 -0700
Message-ID: <CAPcxDJ7=UsAkDwVuoQcTt2B2UA4RWjs_o_=Fnk4Hfuqj+V8hAA@mail.gmail.com>
Subject: Re: [PATCH 2/3] x86/mce: Avoid infinite loop for copy from user recovery
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        "dinghui@sangfor.com.cn" <dinghui@sangfor.com.cn>,
        "huangcun@sangfor.com.cn" <huangcun@sangfor.com.cn>,
        "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= 
        <naoya.horiguchi@nec.com>, Oscar Salvador <osalvador@suse.de>,
        x86 <x86@kernel.org>, "Song, Youquan" <youquan.song@intel.com>
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On Thu, Jul 22, 2021 at 9:01 PM Luck, Tony <tony.luck@intel.com> wrote:
>
> >> I'm not aware of, nor expecting to find, places where the kernel
> >> tries to access user address A and hits poison, and then tries to
> >> access user address B (without returrning to user between access
> >> A and access B).
> >This seems a reasonablely easy scenario.
> >
> > A user space app allocates a buffer of xyz KB/MB/GB.
> >
> > Unfortunately the dimms are bad and multiple cache lines have
> > uncorrectable errors in them on different pages.
> >
> > Then the user space app tries to write the content of the buffer into some
> > file via write(2) from the entire buffer in one go.
>
> Before this patch Linux gets into an infinite loop taking machine
> checks on the first of the poison addresses in the buffer.
>
> With this patch (and also patch 3/3 in this series). There are
> a few machine checks on the first poison address (I think the number
> depends on the alignment of the poison within a page ... but I'm
> not sure). My test code shows 4 machine checks at the same
> address. Then Linux returns a short byte count to the user
> showing how many bytes were actually written to the file.
>
> The fast that there are many more poison lines in the buffer
> beyond the place where the write stopped on the first one is
> irrelevant.
In our test, the application memory was anon.
With 1 UC error injected, the test always passes with the error
recovered and a SIGBUS delivered to user space.

When there are >1 UC errors in buffer, then indefinite mce loop.
>
> [Well, if the second poisoned line is immediately after the first
> you may hit h/w prefetch issues and h/w may signal a fatal
> machine check ... but that's a different problem that s/w could
> only solve with painful LFENCE operations between each 64-bytes
> of the copy]
>
> -Tony