linux-kernel - Re: [PATCH] mm: hwpoison: fix thp split handing in soft_offline_in_use

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Date:   Tue, 26 Feb 2019 22:34:32 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     <n-horiguchi@...jp.nec.com>, <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <mhocko@...e.com>, <hughd@...gle.com>, <mhocko@...nel.org>
Subject: Re: [PATCH] mm: hwpoison: fix thp split handing in soft_offline_in_use_page()

On 2019/2/26 21:51, Kirill A. Shutemov wrote:
> On Tue, Feb 26, 2019 at 07:18:00PM +0800, zhong jiang wrote:
>> From: zhongjiang <zhongjiang@...wei.com>
>>
>> When soft_offline_in_use_page() runs on a thp tail page after pmd is plit,
> s/plit/split/
>
>> we trigger the following VM_BUG_ON_PAGE():
>>
>> Memory failure: 0x3755ff: non anonymous thp
>> __get_any_page: 0x3755ff: unknown zero refcount page type 2fffff80000000
>> Soft offlining pfn 0x34d805 at process virtual address 0x20fff000
>> page:ffffea000d360140 count:0 mapcount:0 mapping:0000000000000000 index:0x1
>> flags: 0x2fffff80000000()
>> raw: 002fffff80000000 ffffea000d360108 ffffea000d360188 0000000000000000
>> raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
>> page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
>> ------------[ cut here ]------------
>> kernel BUG at ./include/linux/mm.h:519!
>>
>> soft_offline_in_use_page() passed refcount and page lock from tail page to
>> head page, which is not needed because we can pass any subpage to
>> split_huge_page().
> I don't see a description of what is going wrong and why change will fixed
> it. From the description, it appears as it's cosmetic-only change.
>
> Please elaborate.
When soft_offline_in_use_page runs on a thp tail page after pmd is split,  
and we pass the head page to split_huge_page, Unfortunately, the tail page
can be free or count turn into zero.

Thanks,
zhong jiang