Message-ID: <04d2d781-fd5e-4778-b042-d4dbeb8c5d49@redhat.com>
Date: Fri, 12 Sep 2025 10:41:10 +0200
From: David Hildenbrand <david@...hat.com>
To: Chunyan Zhang <zhang.lyra@...il.com>
Cc: 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,
 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>
Subject: Re: [PATCH v11 1/5] mm: softdirty: Add pgtable_soft_dirty_supported()

[...]

>>> +/*
>>> + * 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.

The whole hunk will not be required when we make sure VM_SOFTDIRTY never 
gets set, correct?

> 
>>
>> 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?

vm_flags is passed in by the caller?

But just to clarify: this code was just a quick hack, adjust it as you need.

[...]

>>>
>>> +     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.

I meant that there is no need to add the pgtable_soft_dirty_supported() 
check.

> 
> Sorry I'm not sure I understand here, you mean no longer need #ifdef
> CONFIG_MEM_SOFT_DIRTY for these function definitions, right?

Likely we could drop them. VM_SOFTDIRTY will never be set so the code 
will not be invoked.

And for architectures where VM_SOFTDIRTY is never even possible 
(!CONFIG_MEM_SOFT_DIRTY) we keep it as 0.

That way, the compiler can even optimize out all of that code because

"vma->vm_flags & VM_SOFTDIRTY" -> "vma->vm_flags & 0"

will never be true.

> 
>>
>>> 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?
> 

I think we have that already in include/linux/pgtable.h:

We have stubs that just don't do anything.

For riscv support you would handle runtime-enablement in these helpers.

> 
>>
>>>        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.
Hm, let me think about that. No, I think this has to stay as the comment
says, so this case here is special.

-- 
Cheers

David / dhildenb

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