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

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