[PATCH v3] arm64/mm: fix incorrect file_map_count for invalid pmd

[Denys Vlasenko]

On 11/21/22 19:18, Will Deacon wrote:
> On Mon, Nov 21, 2022 at 03:36:08PM +0800, Liu Shixin wrote:
>> The page table check trigger BUG_ON() unexpectedly when split hugepage:
>>
>>   ------------[ cut here ]------------
>>   kernel BUG at mm/page_table_check.c:119!
>>   Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
>>   Dumping ftrace buffer:
>>      (ftrace buffer empty)
>>   Modules linked in:
>>   CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748
>>   Hardware name: linux,dummy-virt (DT)
>>   pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>   pc : page_table_check_set.isra.0+0x398/0x468
>>   lr : page_table_check_set.isra.0+0x1c0/0x468
>> [...]
>>   Call trace:
>>    page_table_check_set.isra.0+0x398/0x468
>>    __page_table_check_pte_set+0x160/0x1c0
>>    __split_huge_pmd_locked+0x900/0x1648
>>    __split_huge_pmd+0x28c/0x3b8
>>    unmap_page_range+0x428/0x858
>>    unmap_single_vma+0xf4/0x1c8
>>    zap_page_range+0x2b0/0x410
>>    madvise_vma_behavior+0xc44/0xe78
>>    do_madvise+0x280/0x698
>>    __arm64_sys_madvise+0x90/0xe8
>>    invoke_syscall.constprop.0+0xdc/0x1d8
>>    do_el0_svc+0xf4/0x3f8
>>    el0_svc+0x58/0x120
>>    el0t_64_sync_handler+0xb8/0xc0
>>    el0t_64_sync+0x19c/0x1a0
>> [...]
>>
>> On arm64, pmd_leaf() will return true even if the pmd is invalid due to
>> pmd_present_invalid() check. So in pmdp_invalidate() the file_map_count
>> will not only decrease once but also increase once. Then in set_pte_at(),
>> the file_map_count increase again, and so trigger BUG_ON() unexpectedly.
>>
>> Add !pmd_present_invalid() check in pmd_user_accessible_page() to fix the
>> problem.
>>
>> Fixes: 42b2547137f5 ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
>> Reported-by: Denys Vlasenko <dvlasenk at redhat.com>
>> Signed-off-by: Liu Shixin <liushixin2 at huawei.com>
>> Acked-by: Pasha Tatashin <pasha.tatashin at soleen.com>
>> Acked-by: David Hildenbrand <david at redhat.com>
>> Reviewed-by: Kefeng Wang <wangkefeng.wang at huawei.com>
>> ---
>> v1->v2: Update comment and optimize the code by moving p?d_valid() at
>> 	first place suggested by Mark.
>> v2->v3: Replace pmd_valid() with pmd_present_invalid() suggested by Will.
>>
>>   arch/arm64/include/asm/pgtable.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index edf6625ce965..17afb09f386f 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -863,7 +863,7 @@ static inline bool pte_user_accessible_page(pte_t pte)
>>   
>>   static inline bool pmd_user_accessible_page(pmd_t pmd)
>>   {
>> -	return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
>> +	return pmd_leaf(pmd) && !pmd_present_invalid(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
>>   }
> 
> Acked-by: Will Deacon <will at kernel.org>
> 
> But please see my comment on v2 about pud_user_exec() for the PUD case.

Can you be more specific? Do you ask for pud_user_exec() to be defined
and used here? Or something else?

Until this patch lands, amd64 PAGE_TABLE_CHECK + THP remains broken...