Re: [v3 PATCH] arm64: mm: Fix kexec failure after pte_mkwrite_novma() change

["Chang, Jianpeng (CN)" <Jianpeng.Chang.CN@windriver.com>]

On 12/4/2025 4:07 PM, Anshuman Khandual wrote:
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> On 04/12/25 11:57 AM, Jianpeng Chang wrote:
>> Commit 143937ca51cc ("arm64, mm: avoid always making PTE dirty in
>> pte_mkwrite()") modified pte_mkwrite_novma() to only clear PTE_RDONLY
>> when the page is already dirty (PTE_DIRTY is set). While this optimization
>> prevents unnecessary dirty page marking in normal memory management paths,
>> it breaks kexec on some platforms like NXP LS1043.
> 
> Why is this problem only applicable for NXP LS1043 ? OR is that the only
> platform you have observed the issue ? although that is problematic else
> where as well.

Not only 1043. I found it on the NXP LS1043, and I have both NXP LS1043 
and LS1046 boards available. They both have this issue.

> 
>>
>> The issue occurs in the kexec code path:
>> 1. machine_kexec_post_load() calls trans_pgd_create_copy() to create a
>>     writable copy of the linear mapping
>> 2. _copy_pte() calls pte_mkwrite_novma() to ensure all pages in the copy
>>     are writable for the new kernel image copying
>> 3. With the new logic, clean pages (without PTE_DIRTY) remain read-only
>> 4. When kexec tries to copy the new kernel image through the linear
>>     mapping, it fails on read-only pages, causing the system to hang
>>     after "Bye!"
>>
>> The same issue affects hibernation which uses the same trans_pgd code path.
>>
>> Fix this by marking pages dirty with pte_mkdirty() in _copy_pte(), which
>> ensures pte_mkwrite_novma() clears PTE_RDONLY for both kexec and
>> hibernation, making all pages in the temporary mapping writable regardless
>> of their dirty state. This preserves the original commit's optimization
>> for normal memory management while fixing the kexec/hibernation regression.
>>
>> Using pte_mkdirty() causes redundant bit operations when the page is
>> already writable (redundant PTE_RDONLY clearing), but this is acceptable
>> since it's not a hot path and only affects kexec/hibernation scenarios.
>>
>> Fixes: 143937ca51cc ("arm64, mm: avoid always making PTE dirty in pte_mkwrite()")
>> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@windriver.com>
>> Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
>> ---
>> v3:
>>    - Add the description about pte_mkdirty in commit message
>>    - Note that the redundant bit operations in commit message
>>    - Fix the comments following the suggestions
>> v2: https://lore.kernel.org/all/20251202022707.2720933-1-jianpeng.chang.cn@windriver.com/
>>    - Use pte_mkwrite_novma(pte_mkdirty(pte)) instead of manual bit manipulation
>>    - Updated comments to clarify pte_mkwrite_novma() alone cannot be used
>> v1: https://lore.kernel.org/all/20251127034350.3600454-1-jianpeng.chang.cn@windriver.com/
>>
>>   arch/arm64/mm/trans_pgd.c | 17 +++++++++++++++--
>>   1 file changed, 15 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
>> index 18543b603c77..766883780d2a 100644
>> --- a/arch/arm64/mm/trans_pgd.c
>> +++ b/arch/arm64/mm/trans_pgd.c
>> @@ -40,8 +40,14 @@ static void _copy_pte(pte_t *dst_ptep, pte_t *src_ptep, unsigned long addr)
>>                 * Resume will overwrite areas that may be marked
>>                 * read only (code, rodata). Clear the RDONLY bit from
>>                 * the temporary mappings we use during restore.
>> +              *
>> +              * For both kexec and hibernation, writable accesses are required
>> +              * for all pages in the linear map to copy over new kernel image.
>> +              * Hence mark these pages dirty first via pte_mkdirty() to ensure
>> +              * pte_mkwrite_novma() subsequently clears PTE_RDONLY - providing
>> +              * required write access for the pages.
>>                 */
>> -             __set_pte(dst_ptep, pte_mkwrite_novma(pte));
>> +             __set_pte(dst_ptep, pte_mkwrite_novma(pte_mkdirty(pte)));
>>        } else if (!pte_none(pte)) {
>>                /*
>>                 * debug_pagealloc will removed the PTE_VALID bit if
>> @@ -57,7 +63,14 @@ static void _copy_pte(pte_t *dst_ptep, pte_t *src_ptep, unsigned long addr)
>>                 */
>>                BUG_ON(!pfn_valid(pte_pfn(pte)));
>>
>> -             __set_pte(dst_ptep, pte_mkvalid(pte_mkwrite_novma(pte)));
>> +             /*
>> +              * For both kexec and hibernation, writable accesses are required
>> +              * for all pages in the linear map to copy over new kernel image.
>> +              * Hence mark these pages dirty first via pte_mkdirty() to ensure
>> +              * pte_mkwrite_novma() subsequently clears PTE_RDONLY - providing
>> +              * required write access for the pages.
>> +              */
>> +             __set_pte(dst_ptep, pte_mkvalid(pte_mkwrite_novma(pte_mkdirty(pte))));
>>        }
>>   }
>>
>