linux-kernel - Re: [PATCH v2 00/12] mm: page migration enhancement for thp

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Date:   Thu, 2 Feb 2017 21:24:03 -0600
From:   Zi Yan <zi.yan@...rutgers.edu>
To:     Hillf Danton <hillf.zj@...baba-inc.com>
CC:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        <linux-mm@...ck.org>, Hugh Dickins <hughd@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Emelyanov <xemul@...allels.com>,
        Balbir Singh <bsingharora@...il.com>,
        <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v2 00/12] mm: page migration enhancement for thp

On 2 Feb 2017, at 21:12, Hillf Danton wrote:

> On February 01, 2017 3:54 AM Zi Yan wrote:
>>
>> I am also doing some tests on THP migration and discover that there are
>> some corner cases not handled in this patchset.
>>
>> For example, in handle_mm_fault, without taking pmd_lock, the kernel may
>> see pmd_none(*pmd) during THP migrations, which leads to
>> handle_pte_fault or even deeper in the code path. At that moment,
>> pmd_trans_unstable() will treat a pmd_migration_entry as pmd_bad and
>> clear it. This leads to application crashing and page table leaks, since
>> a deposited PTE page is not released when the application crashes.
>>
>> Even after I add is_pmd_migration_entry() into pmd_trans_unstable(), I
>> still see application data corruptions.
>>
>> I hope someone can shed some light on how to debug this. Should I also
>> look into pmd_trans_huge() call sites where pmd_migration_entry should
>> be handled differently?
>>
> Hm ... seems it helps more if you post your current works as RFC on
> top of the mm tree, and the relevant tests as well.
>

Thanks for replying. I find that data corruption is caused by that
set_pmd_migration_entry() did not flush TLB while changing pmd entries.
I fix it by using pmdp_huge_clear_flush() instead of pmdp_huge_get_and_clear().

The racy pmd check in zap_pmd_range() also causes pmd_bad() problem. I am going
to rebase Naoya's patches and send it again for comments.



> Hillf
>>
>> Anshuman Khandual wrote:
>>> On 11/08/2016 05:01 AM, Naoya Horiguchi wrote:
>>>> Hi everyone,
>>>>
>>>> I've updated thp migration patches for v4.9-rc2-mmotm-2016-10-27-18-27
>>>> with feedbacks for ver.1.
>>>
>>> Hello Noaya,
>>>
>>> I have been working with Zi Yan on the parallel huge page migration series
>>> (https://lkml.org/lkml/2016/11/22/457) and planning to post them on top of
>>> this THP migration enhancement series. Hence we were wondering if you have
>>> plans to post a new version of this series in near future ?
>>>
>>> Regards
>>> Anshuman
>>>
>>
>> --
>> Best Regards,
>> Yan Zi


--
Best Regards
Yan Zi

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