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Subject: Re: [PATCH -next] mm: hwpoison: support recovery from HugePage
 copy-on-write faults
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20230411092741.780679-1-liushixin2@huawei.com>
 <20230412181350.GA22818@monkey>
 <20230412145718.0bcb7dd98112a3010711ad0b@linux-foundation.org>
 <20230412222138.GB4759@monkey>
CC:     Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Tony Luck <tony.luck@intel.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Muchun Song <muchun.song@linux.dev>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
From:   Liu Shixin <liushixin2@huawei.com>
Message-ID: <6a5f3acb-bbc5-9e36-e194-84ec15b059b5@huawei.com>
Date:   Thu, 13 Apr 2023 09:51:44 +0800
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On 2023/4/13 6:21, Mike Kravetz wrote:
> On 04/12/23 14:57, Andrew Morton wrote:
>> On Wed, 12 Apr 2023 11:13:50 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>
>>> On 04/11/23 17:27, Liu Shixin wrote:
>>>> Patch a873dfe1032a ("mm, hwpoison: try to recover from copy-on write faults")
>>>> introduced a new copy_user_highpage_mc() function, and fix the kernel crash
>>>> when the kernel is copying a normal page as the result of a copy-on-write
>>>> fault and runs into an uncorrectable error. But it doesn't work for HugeTLB.
>>> Andrew asked about user-visible effects.  Perhaps, a better way of
>>> stating this in the commit message might be:
>>>
>>> Commit a873dfe1032a ("mm, hwpoison: try to recover from copy-on write
>>> faults") introduced the routine copy_user_highpage_mc() to gracefully
>>> handle copying of user pages with uncorrectable errors.  Previously,
>>> such copies would result in a kernel crash.  hugetlb has separate code
>>> paths for copy-on-write and does not benefit from the changes made in
>>> commit a873dfe1032a.
> I was just going to suggest adding the line,
>
> Hence, copy-on-write of hugetlb user pages with uncorrectable errors            
> will result in a kernel crash as was the case with 'normal' pages before        
> commit a873dfe1032a.
>
> However, I'm guessing it might be more clear if we start with the
> runtime effects.  Something like:
>
> copy-on-write of hugetlb user pages with uncorrectable errors will result
> in a kernel crash.  This is because the copy is performed in kernel mode
> and in general we can not handle accessing memory with such errors while
> in kernel mode.  Commit a873dfe1032a ("mm, hwpoison: try to recover from
> copy-on write faults") introduced the routine copy_user_highpage_mc() to
> gracefully handle copying of user pages with uncorrectable errors.  However,
> the separate hugetlb copy-on-write code paths were not modified as part
> of commit a873dfe1032a.
Thanks for your advice, I will add these explaination.
>
>>> Modify hugetlb copy-on-write code paths to use copy_mc_user_highpage()
>>> so that they can also gracefully handle uncorrectable errors in user
>>> pages.  This involves changing the hugetlb specific routine
>>> ?copy_user_folio()? from type void to int so that it can return an error.
>>> Modify the hugetlb userfaultfd code in the same way so that it can return
>>> -EHWPOISON if it encounters an uncorrectable error.
>> Thanks, but...  what are the runtime effects?  What does hugetlb
>> presently do when encountering these uncorrectable error?