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Message-ID: <CAAfSe-tQgmBm=RS2gCi7VaRW1XZhS_sJ9rHbvqJ0w=KwTf+m3g@mail.gmail.com>
Date: Fri, 12 Sep 2025 17:21:11 +0800
From: Chunyan Zhang <zhang.lyra@...il.com>
To: David Hildenbrand <david@...hat.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()
On Fri, 12 Sept 2025 at 16:41, David Hildenbrand <david@...hat.com> wrote:
>
> [...]
>
> >>> +/*
> >>> + * 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?
Oh no, this hunk code does not set vmflag.
The mnemonics[ilog2(VM_SOFTDIRTY)] is for show_smap_vma_flags(),
something like below:
# cat /proc/1/smaps
5555605c7000-555560680000 r-xp 00000000 fe:00 19
/bin/busybox
...
VmFlags: rd ex mr mw me sd
'sd' is for soft-dirty
I think this is still needed, right?
>
> >
> >>
> >> 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?
>
Then the original code seems wrong.
> But just to clarify: this code was just a quick hack, adjust it as you need.
Got it.
>
> [...]
>
> >>>
> >>> + 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.
Ok I will take a look.
>
> >
> > 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.
The relationship of VM_SOFTDIRTY and clear_soft_dirty_pmd() is not
very direct from the first sight, let me take a further look.
>
> And for architectures where VM_SOFTDIRTY is never even possible
> (!CONFIG_MEM_SOFT_DIRTY) we keep it as 0.
Ok.
>
> 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.
I will cook a new version and then we can discuss further based on the
new patch.
Thanks for your review,
Chunyan
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