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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/16/22 14:08, 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.

Could you please provide the pmd_val() on the pmd entry, that triggers this
BUG_ON() here ? Only additional thing pmd_leaf() ensures, is that the entry
is not a table one.

#define pmd_leaf(pmd)           (pmd_present(pmd) && !pmd_table(pmd))

collapse_huge_page() pmd is non-leaf because it has table bit on ?

> 
> 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@redhat.com>
> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 71a1af42f0e8..edf6625ce965 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -863,12 +863,12 @@ static inline bool pte_user_accessible_page(pte_t pte)
>  
>  static inline bool pmd_user_accessible_page(pmd_t pmd)
>  {
> -	return pmd_present(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
> +	return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
>  }
>  
>  static inline bool pud_user_accessible_page(pud_t pud)
>  {
> -	return pud_present(pud) && pud_user(pud);
> +	return pud_leaf(pud) && pud_user(pud);
>  }
>  #endif
>