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Date:   Mon, 9 May 2022 15:25:57 -0700
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Subject: Re: [PATCH v2 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when
 unmapping
Content-Language: en-US
To:     Baolin Wang <baolin.wang@linux.alibaba.com>,
        akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org
Cc:     tsbogend@alpha.franken.de, James.Bottomley@HansenPartnership.com,
        deller@gmx.de, mpe@ellerman.id.au, benh@kernel.crashing.org,
        paulus@samba.org, hca@linux.ibm.com, gor@linux.ibm.com,
        agordeev@linux.ibm.com, borntraeger@linux.ibm.com,
        svens@linux.ibm.com, ysato@users.sourceforge.jp, dalias@libc.org,
        davem@davemloft.net, arnd@arndb.de,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-ia64@vger.kernel.org, linux-mips@vger.kernel.org,
        linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org
References: <cover.1652002221.git.baolin.wang@linux.alibaba.com>
 <43b11b69e9f0d9d7e7960b86661db27cc404d0c7.1652002221.git.baolin.wang@linux.alibaba.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
In-Reply-To: <43b11b69e9f0d9d7e7960b86661db27cc404d0c7.1652002221.git.baolin.wang@linux.alibaba.com>
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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 unmapping a hugetlb page, we will get the relevant page table
> entry by huge_pte_offset() only once to nuke it. 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 only one pte or pmd
> entry for this CONT-PTE/PMD size hugetlb page.
> 
> And now try_to_unmap() is only passed a hugetlb page in the case
> where the hugetlb page is poisoned. Which means now we will unmap
> only one pte entry for a CONT-PTE or CONT-PMD size poisoned hugetlb
> page, and we can still access other subpages of a CONT-PTE or CONT-PMD
> size poisoned hugetlb page, which will cause serious issues possibly.
> 
> So we should change to use huge_ptep_clear_flush() to nuke the
> hugetlb page table to fix this issue, which already considered
> CONT-PTE and CONT-PMD size hugetlb.
> 
> We've already used set_huge_swap_pte_at() to set a poisoned
> swap entry for a poisoned hugetlb page. Meanwhile adding a VM_BUG_ON()
> to make sure the passed hugetlb page is poisoned in try_to_unmap().
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/rmap.c | 39 ++++++++++++++++++++++-----------------
>  1 file changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 7cf2408..37c8fd2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1530,6 +1530,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  
>  		if (folio_test_hugetlb(folio)) {
>  			/*
> +			 * The try_to_unmap() is only passed a hugetlb page
> +			 * in the case where the hugetlb page is poisoned.
> +			 */
> +			VM_BUG_ON_PAGE(!PageHWPoison(subpage), subpage);
> +			/*

It is unfortunate that this could not easily be added to the first
if (folio_test_hugetlb(folio)) block in this routine.  However, it
is fine to add here.

Looks good.  Thanks for all these changes,

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

-- 
Mike Kravetz