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

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

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.

> 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));

?

>  	} 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