Message-ID: <9bcaf3ec-c0a1-4ca5-87aa-f84e297d1e42@redhat.com>
Date: Thu, 11 Sep 2025 15:09:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Chunyan Zhang <zhangchunyan@...as.ac.cn>,
 linux-riscv@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, Deepak Gupta <debug@...osinc.com>,
 Ved Shanbhogue <ved@...osinc.com>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
 Arnd Bergmann <arnd@...db.de>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>,
 Chunyan Zhang <zhang.lyra@...il.com>
Subject: Re: [PATCH v11 1/5] mm: softdirty: Add pgtable_soft_dirty_supported()

On 11.09.25 11:55, Chunyan Zhang wrote:
> Some platforms can customize the PTE PMD entry soft-dirty bit making it
> unavailable even if the architecture provides the resource.
> 
> Add an API which architectures can define their specific implementations
> to detect if soft-dirty bit is available on which device the kernel is
> running.

Thinking to myself: maybe pgtable_supports_soft_dirty() would read better
Whatever you prefer.

> 
> Signed-off-by: Chunyan Zhang <zhangchunyan@...as.ac.cn>
> ---
>   fs/proc/task_mmu.c      | 17 ++++++++++++++++-
>   include/linux/pgtable.h | 12 ++++++++++++
>   mm/debug_vm_pgtable.c   | 10 +++++-----
>   mm/huge_memory.c        | 13 +++++++------
>   mm/internal.h           |  2 +-
>   mm/mremap.c             | 13 +++++++------
>   mm/userfaultfd.c        | 10 ++++------
>   7 files changed, 52 insertions(+), 25 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 29cca0e6d0ff..9e8083b6d4cd 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 soft-dirty bit is
> + * unavailable on which the kernel is running, even if the architecture
> + * provides the resource and soft-dirty is compiled in.
> + */
> +#ifdef CONFIG_MEM_SOFT_DIRTY
> +	if (!pgtable_soft_dirty_supported())
> +		mnemonics[ilog2(VM_SOFTDIRTY)][0] = 0;
> +#endif

You can now drop the ifdef.

But, I wonder if could we instead just stop setting the flag. Then we don't
have to worry about any VM_SOFTDIRTY checks.

Something like the following

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 892fe5dbf9de0..8b8bf63a32ef7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -783,6 +783,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
  static inline void vm_flags_init(struct vm_area_struct *vma,
  				 vm_flags_t flags)
  {
+	VM_WARN_ON_ONCE(!pgtable_soft_dirty_supported() && (flags & VM_SOFTDIRTY));
  	ACCESS_PRIVATE(vma, __vm_flags) = flags;
  }
  
@@ -801,6 +802,7 @@ static inline void vm_flags_reset(struct vm_area_struct *vma,
  static inline void vm_flags_reset_once(struct vm_area_struct *vma,
  				       vm_flags_t flags)
  {
+	VM_WARN_ON_ONCE(!pgtable_soft_dirty_supported() && (flags & VM_SOFTDIRTY));
  	vma_assert_write_locked(vma);
  	WRITE_ONCE(ACCESS_PRIVATE(vma, __vm_flags), flags);
  }
@@ -808,6 +810,7 @@ static inline void vm_flags_reset_once(struct vm_area_struct *vma,
  static inline void vm_flags_set(struct vm_area_struct *vma,
  				vm_flags_t flags)
  {
+	VM_WARN_ON_ONCE(!pgtable_soft_dirty_supported() && (flags & VM_SOFTDIRTY));
  	vma_start_write(vma);
  	ACCESS_PRIVATE(vma, __vm_flags) |= flags;
  }
diff --git a/mm/mmap.c b/mm/mmap.c
index 5fd3b80fda1d5..40cb3fbf9a247 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1451,8 +1451,10 @@ static struct vm_area_struct *__install_special_mapping(
  		return ERR_PTR(-ENOMEM);
  
  	vma_set_range(vma, addr, addr + len, 0);
-	vm_flags_init(vma, (vm_flags | mm->def_flags |
-		      VM_DONTEXPAND | VM_SOFTDIRTY) & ~VM_LOCKED_MASK);
+	vm_flags |= mm->def_flags | VM_DONTEXPAND;
+	if (pgtable_soft_dirty_supported())
+		vm_flags |= VM_SOFTDIRTY;
+	vm_flags_init(vma, vm_flags & ~VM_LOCKED_MASK);
  	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
  
  	vma->vm_ops = ops;
diff --git a/mm/vma.c b/mm/vma.c
index abe0da33c8446..16a1ed2a6199c 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2551,7 +2551,8 @@ static void __mmap_complete(struct mmap_state *map, struct vm_area_struct *vma)
  	 * then new mapped in-place (which must be aimed as
  	 * a completely new data area).
  	 */
-	vm_flags_set(vma, VM_SOFTDIRTY);
+	if (pgtable_soft_dirty_supported())
+		vm_flags_set(vma, VM_SOFTDIRTY);
  
  	vma_set_page_prot(vma);
  }
@@ -2819,7 +2820,8 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
  	mm->data_vm += len >> PAGE_SHIFT;
  	if (vm_flags & VM_LOCKED)
  		mm->locked_vm += (len >> PAGE_SHIFT);
-	vm_flags_set(vma, VM_SOFTDIRTY);
+	if (pgtable_soft_dirty_supported())
+		vm_flags_set(vma, VM_SOFTDIRTY);
  	return 0;
  
  mas_store_fail:
diff --git a/mm/vma_exec.c b/mm/vma_exec.c
index 922ee51747a68..c06732a5a620a 100644
--- a/mm/vma_exec.c
+++ b/mm/vma_exec.c
@@ -107,6 +107,7 @@ int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift)
  int create_init_stack_vma(struct mm_struct *mm, struct vm_area_struct **vmap,
  			  unsigned long *top_mem_p)
  {
+	unsigned long flags  = VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
  	int err;
  	struct vm_area_struct *vma = vm_area_alloc(mm);
  
@@ -137,7 +138,9 @@ int create_init_stack_vma(struct mm_struct *mm, struct vm_area_struct **vmap,
  	BUILD_BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
  	vma->vm_end = STACK_TOP_MAX;
  	vma->vm_start = vma->vm_end - PAGE_SIZE;
-	vm_flags_init(vma, VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP);
+	if (pgtable_soft_dirty_supported())
+		flags |= VM_SOFTDIRTY;
+	vm_flags_init(vma, flags);
  	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
  
  	err = insert_vm_struct(mm, vma);


> +
>   	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 (!pgtable_soft_dirty_supported())
> +		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 (!pgtable_soft_dirty_supported())
> +		return;
> +
>   	if (pmd_present(pmd)) {
>   		/* See comment in change_huge_pmd() */
>   		old = pmdp_invalidate(vma, addr, pmdp);

That would all be handled with the above never-set-VM_SOFTDIRTY.

> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 4c035637eeb7..2a3578a4ae4c 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1537,6 +1537,18 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
>   #define arch_start_context_switch(prev)	do {} while (0)
>   #endif
>   
> +/*
> + * Some platforms can customize the PTE soft-dirty bit making it unavailable
> + * even if the architecture provides the resource.
> + * Adding this API allows architectures to add their own checks for the
> + * devices on which the kernel is running.
> + * Note: When overiding it, please make sure the CONFIG_MEM_SOFT_DIRTY
> + * is part of this macro.
> + */
> +#ifndef pgtable_soft_dirty_supported
> +#define pgtable_soft_dirty_supported()	IS_ENABLED(CONFIG_MEM_SOFT_DIRTY)
> +#endif
> +
>   #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>   #ifndef CONFIG_ARCH_ENABLE_THP_MIGRATION
>   static inline pmd_t pmd_swp_mksoft_dirty(pmd_t pmd)
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 830107b6dd08..b32ce2b0b998 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 (!pgtable_soft_dirty_supported())
>   		return;
>   
>   	pr_debug("Validating PTE soft dirty\n");
> @@ -702,7 +702,7 @@ static void __init pte_swap_soft_dirty_tests(struct pgtable_debug_args *args)
>   {
>   	pte_t pte;
>   
> -	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
> +	if (!pgtable_soft_dirty_supported())
>   		return;
>   
>   	pr_debug("Validating PTE swap soft dirty\n");
> @@ -718,7 +718,7 @@ static void __init pmd_soft_dirty_tests(struct pgtable_debug_args *args)
>   {
>   	pmd_t pmd;
>   
> -	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
> +	if (!pgtable_soft_dirty_supported())
>   		return;
>   
>   	if (!has_transparent_hugepage())
> @@ -734,8 +734,8 @@ static void __init pmd_swap_soft_dirty_tests(struct pgtable_debug_args *args)
>   {
>   	pmd_t pmd;
>   
> -	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) ||
> -		!IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION))
> +	if (!pgtable_soft_dirty_supported() ||
> +	    !IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION))
>   		return;
>   
>   	if (!has_transparent_hugepage())
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9c38a95e9f09..218d430a2ec6 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2271,12 +2271,13 @@ static inline int pmd_move_must_withdraw(spinlock_t *new_pmd_ptl,
>   
>   static pmd_t move_soft_dirty_pmd(pmd_t pmd)
>   {
> -#ifdef CONFIG_MEM_SOFT_DIRTY
> -	if (unlikely(is_pmd_migration_entry(pmd)))
> -		pmd = pmd_swp_mksoft_dirty(pmd);
> -	else if (pmd_present(pmd))
> -		pmd = pmd_mksoft_dirty(pmd);
> -#endif
> +	if (pgtable_soft_dirty_supported()) {
> +		if (unlikely(is_pmd_migration_entry(pmd)))
> +			pmd = pmd_swp_mksoft_dirty(pmd);
> +		else if (pmd_present(pmd))
> +			pmd = pmd_mksoft_dirty(pmd);
> +	}
> +

Wondering, should simply the arch take care of that and we can just clal
pmd_swp_mksoft_dirty / pmd_mksoft_dirty?

>   	return pmd;
>   }
>   
> diff --git a/mm/internal.h b/mm/internal.h
> index 45b725c3dc03..c6ca62f8ecf3 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1538,7 +1538,7 @@ static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma)
>   	 * VM_SOFTDIRTY is defined as 0x0, then !(vm_flags & VM_SOFTDIRTY)
>   	 * will be constantly true.
>   	 */
> -	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
> +	if (!pgtable_soft_dirty_supported())
>   		return false;
>   

That should be handled with the above never-set-VM_SOFTDIRTY.

>   	/*
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e618a706aff5..7beb3114dbf5 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -162,12 +162,13 @@ static pte_t move_soft_dirty_pte(pte_t pte)
>   	 * Set soft dirty bit so we can notice
>   	 * in userspace the ptes were moved.
>   	 */
> -#ifdef CONFIG_MEM_SOFT_DIRTY
> -	if (pte_present(pte))
> -		pte = pte_mksoft_dirty(pte);
> -	else if (is_swap_pte(pte))
> -		pte = pte_swp_mksoft_dirty(pte);
> -#endif
> +	if (pgtable_soft_dirty_supported()) {
> +		if (pte_present(pte))
> +			pte = pte_mksoft_dirty(pte);
> +		else if (is_swap_pte(pte))
> +			pte = pte_swp_mksoft_dirty(pte);
> +	}
> +
>   	return pte;
>   }
>   
-- 
Cheers

David / dhildenb

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