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

[Anshuman Khandual <anshuman.khandual@arm.com>]

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.

> 
> 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))));
>  	}
>  }
>