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

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Date:   Thu, 3 Sep 2020 15:54:42 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ralph Campbell <rcampbell@...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>
Subject: Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd_locked() for
 migration PMD

On Wed, Sep 02, 2020 at 09:58:24AM -0700, 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>

Hm. Could you remind me what codepath splits migration PMD? Maybe it
should wait until migration is complete? We could avoid a lot of
complexity this way.

-- 
 Kirill A. Shutemov