linux-kernel - Re: [PATCH v2 2/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when migration

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Message-ID: <4ef41c10-61f4-558e-76b3-3ea517779985@oracle.com>
Date:   Mon, 9 May 2022 14:05:36 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, catalin.marinas@....com, will@...nel.org
Cc:     tsbogend@...ha.franken.de, James.Bottomley@...senPartnership.com,
        deller@....de, mpe@...erman.id.au, benh@...nel.crashing.org,
        paulus@...ba.org, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, ysato@...rs.sourceforge.jp, dalias@...c.org,
        davem@...emloft.net, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 2/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when
 migration

On 5/8/22 02:36, Baolin Wang wrote:
> On some architectures (like ARM64), it can support CONT-PTE/PMD size
> hugetlb, which means it can support not only PMD/PUD size hugetlb:
> 2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
> size specified.
> 
> When migrating a hugetlb page, we will get the relevant page table
> entry by huge_pte_offset() only once to nuke it and remap it with
> a migration pte entry. This is correct for PMD or PUD size hugetlb,
> since they always contain only one pmd entry or pud entry in the
> page table.
> 
> However this is incorrect for CONT-PTE and CONT-PMD size hugetlb,
> since they can contain several continuous pte or pmd entry with
> same page table attributes. So we will nuke or remap only one pte
> or pmd entry for this CONT-PTE/PMD size hugetlb page, which is
> not expected for hugetlb migration. The problem is we can still
> continue to modify the subpages' data of a hugetlb page during
> migrating a hugetlb page, which can cause a serious data consistent
> issue, since we did not nuke the page table entry and set a
> migration pte for the subpages of a hugetlb page.
> 
> To fix this issue, we should change to use huge_ptep_clear_flush()
> to nuke a hugetlb page table, and remap it with set_huge_pte_at()
> and set_huge_swap_pte_at() when migrating a hugetlb page, which
> already considered the CONT-PTE or CONT-PMD size hugetlb.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  mm/rmap.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)

With the addition of !CONFIG_HUGETLB_PAGE stubs,

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz