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

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

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() ?

> 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.
		*/

> -		__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.

> -		__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 ?