Re: [PATCH V10 1/5] mm: softdirty: Add pte_soft_dirty_available()

[David Hildenbrand <david@redhat.com>]

On 09.09.25 11:56, Chunyan Zhang wrote:
> Some platforms can customize the PTE soft dirty bit and make it unavailable
> even if the architecture allows providing the PTE resource.
> 
> Add an API which architectures can define their specific implementations
> to detect if the PTE soft-dirty bit is available, on which the kernel
> is running.
> 
> Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
> ---
>   fs/proc/task_mmu.c      | 17 ++++++++++++++++-
>   include/linux/pgtable.h | 10 ++++++++++
>   mm/debug_vm_pgtable.c   |  9 +++++----
>   mm/huge_memory.c        | 10 ++++++----
>   mm/internal.h           |  2 +-
>   mm/mremap.c             | 10 ++++++----
>   mm/userfaultfd.c        |  6 ++++--
>   7 files changed, 48 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 29cca0e6d0ff..20a609ec1ba6 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1058,7 +1058,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>   	 * -Werror=unterminated-string-initialization warning
>   	 *  with GCC 15
>   	 */
> -	static const char mnemonics[BITS_PER_LONG][3] = {
> +	static char mnemonics[BITS_PER_LONG][3] = {
>   		/*
>   		 * In case if we meet a flag we don't know about.
>   		 */
> @@ -1129,6 +1129,16 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>   		[ilog2(VM_SEALED)] = "sl",
>   #endif
>   	};
> +/*
> + * We should remove the VM_SOFTDIRTY flag if the PTE soft-dirty bit is
> + * unavailable on which the kernel is running, even if the architecture
> + * allows providing the PTE resource and soft-dirty is compiled in.
> + */
> +#ifdef CONFIG_MEM_SOFT_DIRTY
> +	if (!pte_soft_dirty_available())
> +		mnemonics[ilog2(VM_SOFTDIRTY)][0] = 0;
> +#endif
> +
>   	size_t i;
>   
>   	seq_puts(m, "VmFlags: ");
> @@ -1531,6 +1541,8 @@ static inline bool pte_is_pinned(struct vm_area_struct *vma, unsigned long addr,
>   static inline void clear_soft_dirty(struct vm_area_struct *vma,
>   		unsigned long addr, pte_t *pte)
>   {
> +	if (!pte_soft_dirty_available())
> +		return;
>   	/*
>   	 * The soft-dirty tracker uses #PF-s to catch writes
>   	 * to pages, so write-protect the pte as well. See the
> @@ -1566,6 +1578,9 @@ static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
>   {
>   	pmd_t old, pmd = *pmdp;
>   
> +	if (!pte_soft_dirty_available())
> +		return;
> +
>   	if (pmd_present(pmd)) {
>   		/* See comment in change_huge_pmd() */
>   		old = pmdp_invalidate(vma, addr, pmdp);
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 4c035637eeb7..c0e2a6dc69f4 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1538,6 +1538,15 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
>   #endif
>   
>   #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> +
> +/*
> + * Some platforms can customize the PTE soft dirty bit and make it unavailable
> + * even if the architecture allows providing the PTE resource.
> + */
> +#ifndef pte_soft_dirty_available
> +#define pte_soft_dirty_available()	(true)
> +#endif
> +
>   #ifndef CONFIG_ARCH_ENABLE_THP_MIGRATION
>   static inline pmd_t pmd_swp_mksoft_dirty(pmd_t pmd)
>   {
> @@ -1555,6 +1564,7 @@ static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t pmd)
>   }
>   #endif
>   #else /* !CONFIG_HAVE_ARCH_SOFT_DIRTY */
> +#define pte_soft_dirty_available()	(false)
>   static inline int pte_soft_dirty(pte_t pte)
>   {
>   	return 0;
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 830107b6dd08..98ed7e22ccec 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -690,7 +690,7 @@ static void __init pte_soft_dirty_tests(struct pgtable_debug_args *args)
>   {
>   	pte_t pte = pfn_pte(args->fixed_pte_pfn, args->page_prot);
>   
> -	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
> +	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) || !pte_soft_dirty_available())

I suggest that you instead make pte_soft_dirty_available() be false without CONFIG_MEM_SOFT_DIRTY.

e.g., for the default implementation

define pte_soft_dirty_available()	IS_ENABLED(CONFIG_MEM_SOFT_DIRTY)

That way you can avoid some ifefs and cleanup these checks.


But as we do also have PMD soft-dirty support, I guess we would want to call this
something more abstract "pgtable_soft_dirty_available" or "pgtable_soft_dirty_supported"

-- 
Cheers

David / dhildenb