linux-kernel - Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd

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Message-ID: <171f31cd-91e5-5c74-a68a-ec7cbd9bb450@nvidia.com>
Date:   Wed, 2 Sep 2020 15:02:43 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Zi Yan <ziy@...dia.com>
CC:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Jerome Glisse" <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Alistair Popple" <apopple@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...dia.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd_locked() for
 migration PMD


On 9/2/20 2:47 PM, Zi Yan wrote:
> On 2 Sep 2020, at 12:58, Ralph Campbell wrote:
> 
>> A migrating transparent huge page has to already be unmapped. Otherwise,
>> the page could be modified while it is being copied to a new page and
>> data could be lost. The function __split_huge_pmd() checks for a PMD
>> migration entry before calling __split_huge_pmd_locked() leading one to
>> think that __split_huge_pmd_locked() can handle splitting a migrating PMD.
>> However, the code always increments the page->_mapcount and adjusts the
>> memory control group accounting assuming the page is mapped.
>> Also, if the PMD entry is a migration PMD entry, the call to
>> is_huge_zero_pmd(*pmd) is incorrect because it calls pmd_pfn(pmd) instead
>> of migration_entry_to_pfn(pmd_to_swp_entry(pmd)).
>> Fix these problems by checking for a PMD migration entry.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
> 
> Thanks for the fix. You can add Reviewed-by: Zi Yan <ziy@...dia.com>
> 
> I think you also want to add the Fixes tag and cc stable.
> 
> Fixes 84c3fc4e9c56 (“mm: thp: check pmd migration entry in common path”)
> cc: stable@...r.kernel.org # 4.14+

Thanks, I'll add these.