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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Jianpeng Chang <jianpeng.chang.cn@windriver.com> writes:

> On 11/28/25 5:32 PM, Huang, Ying wrote:
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>>
>> Hi, Jianpeng,
>>
>> Jianpeng Chang <jianpeng.chang.cn@windriver.com> writes:
>>
>>> 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
>>> 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()")
>> IMHO, this isn't the right "Fixes" tag.  The original _copy_pte() code
>> should be the fixing target.
>
> Hi Ying,
>
> According to my understanding, the Fixes tag should point to the
> commit that directly
>
> introduced the issue. While _copy_pte() was introduced with
> pte_mkwrite_novma() in
>
> commit 6ecc21bb432d, at that time pte_mkwrite_novma() always cleared
> the PTE_RDONLY

Per my understanding, that is not correct to clearing PTE_RDONLY in
pte_mkwrite_novma() in the first place.

However, "Fixes" flag is used for patch backporting too.  For that, the
original tag is better.  So, It's OK for me now.

> bit unconditionally, and kexec worked correctly. Should we blame a
> change that was
>
> working properly at the time, or am I missing something here?
>
>>
>>> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@windriver.com>
>>> ---
>>>   arch/arm64/mm/trans_pgd.c | 12 ++++++++++--
>>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
>>> index 18543b603c77..ad4e5e4fcc91 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 regardless
>>> +              * of the page's dirty state, so force clear PTE_RDONLY.
>>>                 */
>>> -             __set_pte(dst_ptep, pte_mkwrite_novma(pte));
>>> +             pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
>>> +             pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>>> +             __set_pte(dst_ptep, pte);
>> Why not
>>
>>          __set_pte(dst_ptep, pte_mkwrite_novma(pte_mkdirty(pte));
> I agree that using pte_mkdirty() is indeed better, makes the
> modification clearer and avoids the helper function. I will change it.

Thanks!

>> ?
>>
>>>        } else if (!pte_none(pte)) {
>>>                /*
>>>                 * debug_pagealloc will removed the PTE_VALID bit if
>>> @@ -57,7 +62,10 @@ 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)));
>>> +             pte = pte_mkvalid(pte);
>>> +             pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
>>> +             pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>>> +             __set_pte(dst_ptep, pte);
>>>        }
>>>   }

---
Best Regards,
Huang, Ying