Re: [v2 PATCH] arm64: mm: Fix kexec failure after pte_mkwrite_novma() change

[Jianpeng Chang <jianpeng.chang.cn@windriver.com>]

On 12/2/25 2:57 PM, Anshuman Khandual wrote:
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> On 02/12/25 7:57 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.
>>
>> 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 explicitly clearing PTE_RDONLY in _copy_pte() for both
> via pte_mkdirty() ?
Sorry, for this.

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.
>
>> kexec and hibernation, ensuring all pages in the temporary mapping are
>> writable regardless of their dirty state. This preserves the original
>> commit's optimization for normal memory management while fixing the
>> kexec/hibernation regression.
>>
>> Fixes: 143937ca51cc ("arm64, mm: avoid always making PTE dirty in pte_mkwrite()")
>> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@windriver.com>
>> ---
>> v2:
>>   - 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 | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
>> index 18543b603c77..08f5ee6643e1 100644
>> --- a/arch/arm64/mm/trans_pgd.c
>> +++ b/arch/arm64/mm/trans_pgd.c
>> @@ -40,8 +40,13 @@ 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 kexec/hibernation, we need writable access to all
>> +              * pages in the linear mapping to copy the new kernel image.
>> +              * Mark pages dirty first to ensure pte_mkwrite_novma()
>> +              * clears PTE_RDONLY.
>>                */
>                /*
>                 * 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.
>                 */
I will change it.
>
>> -             __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 +62,7 @@ static void _copy_pte(pte_t *dst_ptep, pte_t *src_ptep, unsigned long addr)
>>                */
>>               BUG_ON(!pfn_valid(pte_pfn(pte)));
>>
> The comments should be replicated here as well given the same special situation.
I will change it.
>
>> -             __set_pte(dst_ptep, pte_mkvalid(pte_mkwrite_novma(pte)));
>> +             __set_pte(dst_ptep, pte_mkvalid(pte_mkwrite_novma(pte_mkdirty(pte))));
>>       }
>>  }
>>
> static inline pte_t pte_mkwrite_novma(pte_t pte)
> {
>         pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
>         if (pte_sw_dirty(pte))
>                 pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>         return pte;
> }
>
> static inline pte_t pte_mkdirty(pte_t pte)
> {
>         pte = set_pte_bit(pte, __pgprot(PTE_DIRTY));
>
>         if (pte_write(pte))
>                 pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>
>         return pte;
> }
>
> So if pte_write() is true, there will be a redundant PTE_RDONLY clearing which is OK.
> Should this be mentioned in the commit message ?
TBH, I don't have a better idea - any suggestions?

Or, let's add some lines:

Using pte_mkdirty() causes redundant bit operations, but this is
acceptable since it's not a hot path.