Re: [PATCH] arm64: dax: add devmap check for pmd_trans_huge

[mawupeng <mawupeng1@huawei.com>]

On 2025/4/9 7:05, Alistair Popple wrote:
> On Tue, Apr 08, 2025 at 04:59:14PM +0800, Wupeng Ma wrote:
>> During our test in ext4 dax linux-v5.10 on arm64. A BUG_ON is trigger in
>> follow_invalidate_pte as follow since this pmd is seem as pmd_trans_huge.
>> However this page is really a dax-pmds rather than a pmd trans huge.
>>
>> Call trace is shown as follow:
>>
>>   ------------[ cut here ]------------
>>   kernel BUG at mm/memory.c:5185!
>>   CPU: 0 PID: 150 Comm: kworker/u8:10 Not tainted 5.10.0-01678-g1e62aad66bbc-dirty #36
> 
> This is an old kernel, and I couldn't correlate the line number of the BUG_ON()
> probably because you have patches applied. But I assume this is the VM_BUG_ON()
> in follow_invalidate_pte()? Does this issue reproduce on more recent kernel

Yes.

> versions (eg. v6.13)? Or some other upstream kernel version?

Since Commit 06083a0921fd ("dax: fix missing writeprotect the pte entry"),
the same issue can not be trigger in the same call trace. However the same
issue may still exist in current kernel.

> 
>>   pc : follow_invalidate_pte+0xdc/0x5e0
>>   lr : follow_invalidate_pte+0xc4/0x5e0
>>   sp : ffffa00012997110
>>   Call trace:
>>    follow_invalidate_pte+0xdc/0x5e0
>>    dax_entry_mkclean+0x250/0x870
>>    dax_writeback_one+0xac/0x380
>>    dax_writeback_mapping_range+0x22c/0x704
>>    ext4_dax_writepages+0x234/0x6e4
>>    do_writepages+0xc8/0x1c0
>>    __writeback_single_inode+0xb8/0x560
>>    writeback_sb_inodes+0x344/0x7a0
>>    wb_writeback+0x1f8/0x6b0
>>    wb_do_writeback+0x194/0x3cc
>>    wb_workfn+0x14c/0x590
>>    process_one_work+0x470/0xa30
>>    worker_thread+0xac/0x510
>>    kthread+0x1e0/0x220
>>    ret_from_fork+0x10/0x18
>>   ---[ end trace 0f479050bd4b1818 ]---
>>   Kernel panic - not syncing: Oops - BUG: Fatal exception
>>   ---[ end Kernel panic - not syncing: Oops - BUG: Fatal exception ]---
>>
>> Commit 5c7fb56e5e3f ("mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmd") and
>> commit 36b78402d97a ("powerpc/hash64/devmap: Use H_PAGE_THP_HUGE when
>> setting up huge devmap PTE entries") already check pmd_devmap during
>> checking pmd_trans_huge. Since pmd_devmap() is used to distinguish dax-pmds,
>> add the same check for arm64 to fix this problem.
> 
> That seems correct to me. In practice most callers of pmd_trans_huge() that can
> see a dax-pmd already check for it explicitly with vma_is_dax(), but there are a
> few cases that don't.
> 
>> Add PTE_DEVMAP in pte_modify as commit 4628a64591e6 ("mm: Preserve
>> _PAGE_DEVMAP across mprotect() calls") does to avoid the same issue in
>> mprotect.
>>
>> Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
>> Signed-off-by: Wupeng Ma <mawupeng1@huawei.com>
>> ---
>>  arch/arm64/include/asm/pgtable.h | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index d3b538be1500b..b9a618127c01b 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -740,7 +740,7 @@ static inline int pmd_trans_huge(pmd_t pmd)
>>  	 * as a table, so force the valid bit for the comparison.
>>  	 */
>>  	return pmd_val(pmd) && pmd_present(pmd) &&
>> -	       !pmd_table(__pmd(pmd_val(pmd) | PTE_VALID));
>> +	       !pmd_table(__pmd(pmd_val(pmd) | PTE_VALID)) && !pmd_devmap(pmd);
>>  }
>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>  
>> @@ -1186,7 +1186,8 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>>  	 */
>>  	const pteval_t mask = PTE_USER | PTE_PXN | PTE_UXN | PTE_RDONLY |
>>  			      PTE_PRESENT_INVALID | PTE_VALID | PTE_WRITE |
>> -			      PTE_GP | PTE_ATTRINDX_MASK | PTE_PO_IDX_MASK;
>> +			      PTE_GP | PTE_ATTRINDX_MASK | PTE_PO_IDX_MASK |
>> +			      PTE_DEVMAP;
>>  
>>  	/* preserve the hardware dirty information */
>>  	if (pte_hw_dirty(pte))
>> -- 
>> 2.43.0
>>
>>