linux-kernel - Re: [PATCH 1/2] arm64/mm: fix incorrect file_map

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Date:   Mon, 21 Nov 2022 16:57:37 +0100
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Liu Shixin <liushixin2@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Hildenbrand <dhildenb@...hat.com>,
        Rafael Aquini <raquini@...hat.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64/mm: fix incorrect file_map_count for non-leaf
 pmd/pud

On 11/16/22 09:38, Liu Shixin wrote:
> The page table check trigger BUG_ON() unexpectedly when collapse hugepage:
> 
>   ------------[ cut here ]------------
>   kernel BUG at mm/page_table_check.c:82!
>   Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
>   Dumping ftrace buffer:
>      (ftrace buffer empty)
>   Modules linked in:
>   CPU: 6 PID: 68 Comm: khugepaged Not tainted 6.1.0-rc3+ #750
>   Hardware name: linux,dummy-virt (DT)
>   pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>   pc : page_table_check_clear.isra.0+0x258/0x3f0
>   lr : page_table_check_clear.isra.0+0x240/0x3f0
> [...]
>   Call trace:
>    page_table_check_clear.isra.0+0x258/0x3f0
>    __page_table_check_pmd_clear+0xbc/0x108
>    pmdp_collapse_flush+0xb0/0x160
>    collapse_huge_page+0xa08/0x1080
>    hpage_collapse_scan_pmd+0xf30/0x1590
>    khugepaged_scan_mm_slot.constprop.0+0x52c/0xac8
>    khugepaged+0x338/0x518
>    kthread+0x278/0x2f8
>    ret_from_fork+0x10/0x20
> [...]
> 
> Since pmd_user_accessible_page() doesn't check if a pmd is leaf, it
> decrease file_map_count for a non-leaf pmd comes from collapse_huge_page().
> and so trigger BUG_ON() unexpectedly.
> 
> Fix this problem by using pmd_leaf() insteal of pmd_present() in
> pmd_user_accessible_page(). Moreover, use pud_leaf() for
> pud_user_accessible_page() too.
> 
> Fixes: 42b2547137f5 ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
> Reported-by: Denys Vlasenko <dvlasenk@...hat.com>
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>


Tested on 6.0.6 kernel, no oopses anymore.