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Subject: Re: [PATCH -next] mm: hwpoison: support recovery from HugePage
 copy-on-write faults
To:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>
References: <20230411092741.780679-1-liushixin2@huawei.com>
 <20230412181350.GA22818@monkey>
 <20230412145718.0bcb7dd98112a3010711ad0b@linux-foundation.org>
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: <28bf1701-d2c6-ee2a-d92d-a603e1a1b3dd@huawei.com>
Date:   Thu, 13 Apr 2023 09:49:31 +0800
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On 2023/4/13 5:57, 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.
>>
>> 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?
I have tested the HugeTLB case by using tony's testcase[1](need add a MAP_HUGETLB).
Before this patch, the kernel will crash due to the uncorrectable errors. After this patch,
if the error occurs in copy-on-write, the process will be killed, if the errors occurs in
userfaultfd, it will return -EHWPOISON.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/aegl/ras-tools.git [1]
>
>
> .
>