Hi David,
On Thu, 11 Sept 2025 at 21:09, David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> 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.
I will use pgtable_supports_* in the next version.
> >
> > Signed-off-by: Chunyan Zhang <zhangchunyan@xxxxxxxxxxx>
> > ---
> > 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.
Ok, you mean define VM_SOFTDIRTY 0x08000000 no matter if
MEM_SOFT_DIRTY is compiled in, right?
Then I need memcpy() to set mnemonics[ilog2(VM_SOFTDIRTY)] here.
>
> 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;
Why use '|=' rather than not directly setting vm_flags which is an
uninitialized variable?
> + 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.
Sorry I'm not sure I understand here, you mean no longer need #ifdef
CONFIG_MEM_SOFT_DIRTY for these function definitions, right?
>
> > 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?
Ok, I think I can do that in another patchset.
>
> > 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.
We don't need to check if (!pgtable_soft_dirty_supported()) if I
understand correctly.
Thanks for the review,
Chunyan
>
> > /*
> > 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
>
