Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
From:   Jue Wang <juew@google.com>
Date:   Mon, 19 Apr 2021 14:28:28 -0700
Message-ID: <CAPcxDJ6SgSagJrF7u576WUb6p7Hg7+beYVoCpJ86Ocsb-mCHmQ@mail.gmail.com>
Subject: Re: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery
To:     tony.luck@intel.com
Cc:     bp@alien8.de, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        luto@kernel.org, naoya.horiguchi@nec.com, x86@kernel.org,
        yaoaili@kingsoft.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, 25 Mar 2021 17:02:35 -0700, Tony Luck wrote:
...

> But there are places in the kernel where the code assumes that this
> EFAULT return was simply because of a page fault. The code takes some
> action to fix that, and then retries the access. This results in a second
> machine check.

What about return EHWPOISON instead of EFAULT and update the callers
to handle EHWPOISON explicitly: i.e., not retry but give up on the page?

My main concern is that the strong assumptions that the kernel can't hit more
than a fixed number of poisoned cache lines before turning to user space
may simply not be true.

When DIMM goes bad, it can easily affect an entire bank or entire ram device
chip. Even with memory interleaving, it's possible that a kernel control path
touches lots of poisoned cache lines in the buffer it is working through.