Re: [PATCH 1/2] arm64/mm: fix incorrect file_map_count for non-leaf pmd/pud

[Denys Vlasenko]

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@xxxxxxxxxx>
> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>


Tested on 6.0.6 kernel, no oopses anymore.