[PATCH] mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()

[PATCH] mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()

[SJ Park]

damon_hugetlb_mkold() reads the page table entry into a local variable,
unsets the accessed bit in the variable, and updates the page table
entry with the updated variable value.  If hardware updates the same
page table entry in parallel, the hw updates could be lost.  For
example, hardware-updated dirty bits might be lost.

Avoid the parallel updates by clearing the page table entry when reading
it together, using huge_ptep_get_and_clear().  If a parallel write to
the memory is made after the clearing, the hw will see the page table
entry is cleared, trigger page fault and wait until it is handled.  The
page fault handling will wait for damon_hugetlb_mkold() due to the page
table lock.

Because hugetlbfs is an in-memory file system and hugetlb pages cannot
be reclaimed, no critical issue is expected to my best knowledge.  But
definitely this is a nasty bug that should be fixed sooner rather than
later.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260830160545.98969-1-sj@kernel.org

Changes from RFC
- RFC: https://lore.kernel.org/20260906195417.103263-1-sj@kernel.org
- Drop RFC tag.

Fixes: 49f4203aae06 ("mm/damon: add access checking for hugetlb pages")
Cc: <stable@vger.kernel.org> # 5.17.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/vaddr.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index f884d3f78f30a..91a0d441c1f94 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -283,22 +283,29 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
 }
 
 #ifdef CONFIG_HUGETLB_PAGE
+static bool damon_hugetlb_ptep_mkold(pte_t *pte, struct mm_struct *mm,
+		struct vm_area_struct *vma, unsigned long addr, pte_t *entry)
+{
+	unsigned long psize = huge_page_size(hstate_vma(vma));
+
+	if (!pte_young(*entry))
+		return false;
+	*entry = huge_ptep_get_and_clear(mm, addr, pte, psize);
+	*entry = pte_mkold(*entry);
+	set_huge_pte_at(mm, addr, pte, *entry, psize);
+	return true;
+}
+
 static void damon_hugetlb_mkold(pte_t *pte, struct mm_struct *mm,
 				struct vm_area_struct *vma, unsigned long addr)
 {
 	bool referenced = false;
 	pte_t entry = huge_ptep_get(mm, addr, pte);
 	struct folio *folio = pfn_folio(pte_pfn(entry));
-	unsigned long psize = huge_page_size(hstate_vma(vma));
 
 	folio_get(folio);
 
-	if (pte_young(entry)) {
-		referenced = true;
-		entry = pte_mkold(entry);
-		set_huge_pte_at(mm, addr, pte, entry, psize);
-	}
-
+	referenced = damon_hugetlb_ptep_mkold(pte, mm, vma, addr, &entry);
 	if (mmu_notifier_clear_young(mm, addr,
 				     addr + huge_page_size(hstate_vma(vma))))
 		referenced = true;

base-commit: 13eba1898080a8a54c60710322909fb0a4d35e84
-- 
2.47.3

Re: [PATCH] mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Modifying contiguous huge PTEs by clearing and re-setting without an intermediate TLB flush violates Break-Before-Make (BBM) rules on architectures like arm64, potentially causing fatal TLB conflict aborts.
--

commit 1fee0c4237cb61938cdabe61cd4adf713c7ddbec
Author: SJ Park <sj@kernel.org>

mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()

This commit attempts to prevent hardware-driven PTE updates (like dirty bits)
from being lost during DAMON's mkold operation on hugetlb pages. It does this
by replacing huge_ptep_get() with huge_ptep_get_and_clear() before calling
set_huge_pte_at().

Link: https://lore.kernel.org/20260830160545.98969-1-sj@kernel.org
Link: https://lore.kernel.org/20260906195417.103263-1-sj@kernel.org

> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index f884d3f78f30a..91a0d441c1f94 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c

[ ... ]

> @@ -283,22 +283,29 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
>  }
>  
>  #ifdef CONFIG_HUGETLB_PAGE
> +static bool damon_hugetlb_ptep_mkold(pte_t *pte, struct mm_struct *mm,
> +		struct vm_area_struct *vma, unsigned long addr, pte_t *entry)
> +{
> +	unsigned long psize = huge_page_size(hstate_vma(vma));
> +
> +	if (!pte_young(*entry))
> +		return false;
> +	*entry = huge_ptep_get_and_clear(mm, addr, pte, psize);
> +	*entry = pte_mkold(*entry);
> +	set_huge_pte_at(mm, addr, pte, *entry, psize);

[Severity: High]
Does this modification of contiguous huge PTEs violate Break-Before-Make (BBM)
rules on architectures like arm64?

By calling huge_ptep_get_and_clear() followed by set_huge_pte_at() without an
intermediate TLB flush, the hardware might cache conflicting contiguous TLB
entries, which could cause fatal TLB conflict aborts. On arm64,
huge_ptep_get_and_clear() clears the constituent PTEs but does not flush the
TLB. When set_huge_pte_at() is subsequently called, it skips its internal BBM
TLB flush because the PTEs were already cleared and its internal validity
check evaluates to false.

> +	return true;
> +}
> +
>  static void damon_hugetlb_mkold(pte_t *pte, struct mm_struct *mm,

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260907170358.100168-1-sj@kernel.org?part=1

Re: [PATCH] mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()

[SJ Park]

On Mon, 07 Sep 2026 17:16:13 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Modifying contiguous huge PTEs by clearing and re-setting without an intermediate TLB flush violates Break-Before-Make (BBM) rules on architectures like arm64, potentially causing fatal TLB conflict aborts.
> --
> 
> commit 1fee0c4237cb61938cdabe61cd4adf713c7ddbec
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()
> 
> This commit attempts to prevent hardware-driven PTE updates (like dirty bits)
> from being lost during DAMON's mkold operation on hugetlb pages. It does this
> by replacing huge_ptep_get() with huge_ptep_get_and_clear() before calling
> set_huge_pte_at().
> 
> Link: https://lore.kernel.org/20260830160545.98969-1-sj@kernel.org
> Link: https://lore.kernel.org/20260906195417.103263-1-sj@kernel.org
> 
> > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> > index f884d3f78f30a..91a0d441c1f94 100644
> > --- a/mm/damon/vaddr.c
> > +++ b/mm/damon/vaddr.c
> 
> [ ... ]
> 
> > @@ -283,22 +283,29 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
> >  }
> >  
> >  #ifdef CONFIG_HUGETLB_PAGE
> > +static bool damon_hugetlb_ptep_mkold(pte_t *pte, struct mm_struct *mm,
> > +		struct vm_area_struct *vma, unsigned long addr, pte_t *entry)
> > +{
> > +	unsigned long psize = huge_page_size(hstate_vma(vma));
> > +
> > +	if (!pte_young(*entry))
> > +		return false;
> > +	*entry = huge_ptep_get_and_clear(mm, addr, pte, psize);
> > +	*entry = pte_mkold(*entry);
> > +	set_huge_pte_at(mm, addr, pte, *entry, psize);
> 
> [Severity: High]
> Does this modification of contiguous huge PTEs violate Break-Before-Make (BBM)
> rules on architectures like arm64?
> 
> By calling huge_ptep_get_and_clear() followed by set_huge_pte_at() without an
> intermediate TLB flush, the hardware might cache conflicting contiguous TLB
> entries, which could cause fatal TLB conflict aborts. On arm64,
> huge_ptep_get_and_clear() clears the constituent PTEs but does not flush the
> TLB. When set_huge_pte_at() is subsequently called, it skips its internal BBM
> TLB flush because the PTEs were already cleared and its internal validity
> check evaluates to false.

But we are changing only accessed bit.  I think BBM is not required for this?
Please correct me if I'm wrong.

> 
> > +	return true;
> > +}
> > +
> >  static void damon_hugetlb_mkold(pte_t *pte, struct mm_struct *mm,
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260907170358.100168-1-sj@kernel.org?part=1


Thanks,
SJ

Re: [PATCH] mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()

[SJ Park]

On Mon,  7 Sep 2026 10:03:56 -0700 SJ Park <sj@kernel.org> wrote:

[...]
> +static bool damon_hugetlb_ptep_mkold(pte_t *pte, struct mm_struct *mm,
> +		struct vm_area_struct *vma, unsigned long addr, pte_t *entry)
> +{
> +	unsigned long psize = huge_page_size(hstate_vma(vma));
> +
> +	if (!pte_young(*entry))
> +		return false;
> +	*entry = huge_ptep_get_and_clear(mm, addr, pte, psize);
> +	*entry = pte_mkold(*entry);
> +	set_huge_pte_at(mm, addr, pte, *entry, psize);
> +	return true;

Sashiko says [1] this might violate BBM.  But this is only for accessed bit
change.  So I don't think BBM is required here.  Also, we intentionally do not
flush TLB because this is just a best-effort monitoring.  Please correct me if
I'm wrong.

[1] https://lore.kernel.org/20260907171613.A09B11F00A3A@smtp.kernel.org


Thanks,
SJ

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