[PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Kefeng Wang]

pte_pfn() and pte_dirty() have undefined behaviour when called on a
non-present PTE. In migrate_vma_collect_pmd(), these functions may be
invoked on non-present entries (e.g., swap PTEs), leading to potential
crashes from pte_pfn() or incorrect dirty folio accounting from
pte_dirty(). Fix both by guarding with pte_present() checks.

Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 mm/migrate_device.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 052167f9ad54..6e711381f092 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -411,7 +411,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 			bool anon_exclusive;
 			pte_t swp_pte;
 
-			flush_cache_page(vma, addr, pte_pfn(pte));
+			if (pte_present(pte))
+				flush_cache_page(vma, addr, pte_pfn(pte));
 			anon_exclusive = folio_test_anon(folio) &&
 					  PageAnonExclusive(page);
 			if (anon_exclusive) {
@@ -432,7 +433,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 			migrate->cpages++;
 
 			/* Set the dirty flag on the folio now the pte is gone. */
-			if (pte_dirty(pte))
+			if (pte_present(pte) && pte_dirty(pte))
 				folio_mark_dirty(folio);
 
 			/* Setup special migration page table entry */
-- 
2.27.0

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Andrew Morton]

On Mon, 6 Jul 2026 19:19:58 +0800 Kefeng Wang <wangkefeng.wang@huawei.com> wrote:

> pte_pfn() and pte_dirty() have undefined behaviour when called on a
> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
> invoked on non-present entries (e.g., swap PTEs), leading to potential
> crashes from pte_pfn() or incorrect dirty folio accounting from
> pte_dirty(). Fix both by guarding with pte_present() checks.
>
> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")

Should we cc:stable?

Sashiko might have found a pre-existing issue:
	https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@huawei.com

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Zi Yan]

On Mon Jul 6, 2026 at 2:01 PM EDT, Andrew Morton wrote:
> On Mon, 6 Jul 2026 19:19:58 +0800 Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>
>> pte_pfn() and pte_dirty() have undefined behaviour when called on a
>> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
>> invoked on non-present entries (e.g., swap PTEs), leading to potential
>> crashes from pte_pfn() or incorrect dirty folio accounting from
>> pte_dirty(). Fix both by guarding with pte_present() checks.
>>
>> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
>> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")
>
> Should we cc:stable?
>
> Sashiko might have found a pre-existing issue:
> 	https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@huawei.com

I looked into the issue and came up with a fix below. It looks ugly, so
feel free to make a better one out of it.

From 7fa06512638a0febb79a00dba63f12818a85788b Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Mon, 6 Jul 2026 21:25:29 -0400
Subject: [PATCH] mm/migrate_device: avoid scanning/collecting PFNs within a
 PMD

migrate_vma_collect_pmd() can drop pte lock to split a large folio and
restart. But the code does not handle restart properly and can cause
collecting inconsistent PFNs and overflowing migrate->dst and migrate->src
arrays. Fix it by:
1. avoiding migrate_vma_collect_huge_pmd() if collection is in progress,
2. skipping the rest of the range if pmd no longer points to a pte page.

Fixes: a30b48bf1b244 ("mm/migrate_device: implement THP migration of zone device pages")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@huawei.com
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/migrate_device.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 052167f9ad547..d86b3989d0c59 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -257,7 +257,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 	pte_t *ptep;
 
 again:
-	if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) {
+	/*
+	 * Only check pmd when addr is at start, namely no pte is scanned or
+	 * collected, to avoid collecting inconsistent PFNs.
+	 */
+	if (addr == start && (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp))) {
 		int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio);
 
 		if (ret == -EAGAIN)
@@ -267,8 +271,18 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 	}
 
 	ptep = pte_offset_map_lock(mm, pmdp, start, &ptl);
-	if (!ptep)
+	if (!ptep) {
+		/*
+		 * Skip the rest if pmd becomes huge or cleared. Flush if any
+		 * pte is modified
+		 */
+		if (addr != start) {
+			if (unmapped)
+				flush_tlb_range(walk->vma, start, end);
+			return migrate_vma_collect_skip(addr, end, walk);
+		}
 		goto again;
+	}
 	lazy_mmu_mode_enable();
 	ptep += (addr - start) / PAGE_SIZE;
 
-- 
2.53.0



-- 
Best Regards,
Yan, Zi

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Kefeng Wang]

On 7/7/2026 9:45 AM, Zi Yan wrote:
> On Mon Jul 6, 2026 at 2:01 PM EDT, Andrew Morton wrote:
>> On Mon, 6 Jul 2026 19:19:58 +0800 Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>
>>> pte_pfn() and pte_dirty() have undefined behaviour when called on a
>>> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
>>> invoked on non-present entries (e.g., swap PTEs), leading to potential
>>> crashes from pte_pfn() or incorrect dirty folio accounting from
>>> pte_dirty(). Fix both by guarding with pte_present() checks.
>>>
>>> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
>>> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")
>>
>> Should we cc:stable?

Yes.


>>
>> Sashiko might have found a pre-existing issue:
>> 	https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@huawei.com
> 
> I looked into the issue and came up with a fix below. It looks ugly, so
> feel free to make a better one out of it.
> 
>  From 7fa06512638a0febb79a00dba63f12818a85788b Mon Sep 17 00:00:00 2001
> From: Zi Yan <ziy@nvidia.com>
> Date: Mon, 6 Jul 2026 21:25:29 -0400
> Subject: [PATCH] mm/migrate_device: avoid scanning/collecting PFNs within a
>   PMD
> 
> migrate_vma_collect_pmd() can drop pte lock to split a large folio and
> restart. But the code does not handle restart properly and can cause
> collecting inconsistent PFNs and overflowing migrate->dst and migrate->src
> arrays. Fix it by:
> 1. avoiding migrate_vma_collect_huge_pmd() if collection is in progress,
> 2. skipping the rest of the range if pmd no longer points to a pte page.
> 
> Fixes: a30b48bf1b244 ("mm/migrate_device: implement THP migration of zone device pages")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@huawei.com
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Zi Yan <ziy@nvidia.com>


Thank you for your fix, it looks good, there is some repetition (3
times) regarding flush+skip in migrate_vma_collect_pmd(), but for bug
fixes, keep it minimal is better. I don't have new ideas to improve it.

> ---
>   mm/migrate_device.c | 18 ++++++++++++++++--
>   1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 052167f9ad547..d86b3989d0c59 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -257,7 +257,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>   	pte_t *ptep;
>   
>   again:
> -	if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) {
> +	/*
> +	 * Only check pmd when addr is at start, namely no pte is scanned or
> +	 * collected, to avoid collecting inconsistent PFNs.
> +	 */
> +	if (addr == start && (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp))) {
>   		int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio);
>   
>   		if (ret == -EAGAIN)
> @@ -267,8 +271,18 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>   	}
>   
>   	ptep = pte_offset_map_lock(mm, pmdp, start, &ptl);
> -	if (!ptep)
> +	if (!ptep) {
> +		/*
> +		 * Skip the rest if pmd becomes huge or cleared. Flush if any
> +		 * pte is modified
> +		 */
> +		if (addr != start) {
> +			if (unmapped)
> +				flush_tlb_range(walk->vma, start, end);
> +			return migrate_vma_collect_skip(addr, end, walk);
> +		}
>   		goto again;
> +	}
>   	lazy_mmu_mode_enable();
>   	ptep += (addr - start) / PAGE_SIZE;
>   

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[David Hildenbrand (Arm)]

On 7/6/26 13:19, Kefeng Wang wrote:
> pte_pfn() and pte_dirty() have undefined behaviour when called on a
> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
> invoked on non-present entries (e.g., swap PTEs), leading to potential

Certainly not real swap PTEs, as the previous function handling is

	if (!pte_present(pte)) {
		...
		if (!softleaf_is_device_private(entry))
			goto next;

Only device-private entries?


> crashes from pte_pfn() or incorrect dirty folio accounting from
> pte_dirty(). Fix both by guarding with pte_present() checks.
> 
> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")

I suspected device-private code, but indeed, these seem to be added afterwards.

But a3589e1d5fe3 only moved the flush_cache_page()? So that one seems wrong.

> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  mm/migrate_device.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 052167f9ad54..6e711381f092 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -411,7 +411,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			bool anon_exclusive;
>  			pte_t swp_pte;
>  
> -			flush_cache_page(vma, addr, pte_pfn(pte));
> +			if (pte_present(pte))
> +				flush_cache_page(vma, addr, pte_pfn(pte));
>  			anon_exclusive = folio_test_anon(folio) &&
>  					  PageAnonExclusive(page);
>  			if (anon_exclusive) {
> @@ -432,7 +433,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			migrate->cpages++;
>  
>  			/* Set the dirty flag on the folio now the pte is gone. */
> -			if (pte_dirty(pte))
> +			if (pte_present(pte) && pte_dirty(pte))
>  				folio_mark_dirty(folio);
>  
>  			/* Setup special migration page table entry */

Changes look good.

-- 
Cheers,

David

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Kefeng Wang]

On 7/7/2026 2:30 AM, David Hildenbrand (Arm) wrote:
> On 7/6/26 13:19, Kefeng Wang wrote:
>> pte_pfn() and pte_dirty() have undefined behaviour when called on a
>> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
>> invoked on non-present entries (e.g., swap PTEs), leading to potential
> 
> Certainly not real swap PTEs, as the previous function handling is
> 
> 	if (!pte_present(pte)) {
> 		...
> 		if (!softleaf_is_device_private(entry))
> 			goto next;
> 
> Only device-private entries?

Yes，device-private entries.

> 
> 
>> crashes from pte_pfn() or incorrect dirty folio accounting from
>> pte_dirty(). Fix both by guarding with pte_present() checks.
>>
>> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
>> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")
> 
> I suspected device-private code, but indeed, these seem to be added afterwards.
> 
> But a3589e1d5fe3 only moved the flush_cache_page()? So that one seems wrong.

I couldn't find a more suitable commit, before a3589e1d5fe3, the
flush_cache_page() only called under anon_exclusive, not a device
private entry, so there is no issue. This is the reason why I chose
this commit.


> 
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/migrate_device.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 052167f9ad54..6e711381f092 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -411,7 +411,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>   			bool anon_exclusive;
>>   			pte_t swp_pte;
>>   
>> -			flush_cache_page(vma, addr, pte_pfn(pte));
>> +			if (pte_present(pte))
>> +				flush_cache_page(vma, addr, pte_pfn(pte));
>>   			anon_exclusive = folio_test_anon(folio) &&
>>   					  PageAnonExclusive(page);
>>   			if (anon_exclusive) {
>> @@ -432,7 +433,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>   			migrate->cpages++;
>>   
>>   			/* Set the dirty flag on the folio now the pte is gone. */
>> -			if (pte_dirty(pte))
>> +			if (pte_present(pte) && pte_dirty(pte))
>>   				folio_mark_dirty(folio);
>>   
>>   			/* Setup special migration page table entry */
> 
> Changes look good.
> 

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[David Hildenbrand (Arm)]

On 7/7/26 03:28, Kefeng Wang wrote:
> 
> 
> On 7/7/2026 2:30 AM, David Hildenbrand (Arm) wrote:
>> On 7/6/26 13:19, Kefeng Wang wrote:
>>> pte_pfn() and pte_dirty() have undefined behaviour when called on a
>>> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
>>> invoked on non-present entries (e.g., swap PTEs), leading to potential
>>
>> Certainly not real swap PTEs, as the previous function handling is
>>
>>     if (!pte_present(pte)) {
>>         ...
>>         if (!softleaf_is_device_private(entry))
>>             goto next;
>>
>> Only device-private entries?
> 
> Yes，device-private entries.
> 
>>
>>
>>> crashes from pte_pfn() or incorrect dirty folio accounting from
>>> pte_dirty(). Fix both by guarding with pte_present() checks.
>>>
>>> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
>>> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")
>>
>> I suspected device-private code, but indeed, these seem to be added afterwards.
>>
>> But a3589e1d5fe3 only moved the flush_cache_page()? So that one seems wrong.
> 
> I couldn't find a more suitable commit, before a3589e1d5fe3, the
> flush_cache_page() only called under anon_exclusive, not a device
> private entry, so there is no issue. This is the reason why I chose
> this commit.

What makes you believe device-private entries wouldn't have PAE set?

This likely goes back to initial support for device-private.

-- 
Cheers,

David

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Kefeng Wang]

On 7/7/2026 3:04 PM, David Hildenbrand (Arm) wrote:
> On 7/7/26 03:28, Kefeng Wang wrote:
>>
>>
>> On 7/7/2026 2:30 AM, David Hildenbrand (Arm) wrote:
>>> On 7/6/26 13:19, Kefeng Wang wrote:
>>>> pte_pfn() and pte_dirty() have undefined behaviour when called on a
>>>> non-present PTE. In migrate_vma_collect_pmd(), these functions may be
>>>> invoked on non-present entries (e.g., swap PTEs), leading to potential
>>>
>>> Certainly not real swap PTEs, as the previous function handling is
>>>
>>>      if (!pte_present(pte)) {
>>>          ...
>>>          if (!softleaf_is_device_private(entry))
>>>              goto next;
>>>
>>> Only device-private entries?
>>
>> Yes，device-private entries.
>>
>>>
>>>
>>>> crashes from pte_pfn() or incorrect dirty folio accounting from
>>>> pte_dirty(). Fix both by guarding with pte_present() checks.
>>>>
>>>> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page")
>>>> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_page()")
>>>
>>> I suspected device-private code, but indeed, these seem to be added afterwards.
>>>
>>> But a3589e1d5fe3 only moved the flush_cache_page()? So that one seems wrong.
>>
>> I couldn't find a more suitable commit, before a3589e1d5fe3, the
>> flush_cache_page() only called under anon_exclusive, not a device
>> private entry, so there is no issue. This is the reason why I chose
>> this commit.
> 
> What makes you believe device-private entries wouldn't have PAE set?
> 
> This likely goes back to initial support for device-private.
> 

We only need to make sure that flush_cache_page(..,pte_pfn(pte)) won't
be called under no-present pte, I go back and find the
flush_cache_page function was first introduced to
migrate_vma_collect_pmd in commit 6c287605fd56, so maybe this commit is
better for fix tag? what's your option?

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[David Hildenbrand (Arm)]

On 7/7/26 11:33, Kefeng Wang wrote:
> 
> 
> On 7/7/2026 3:04 PM, David Hildenbrand (Arm) wrote:
>> On 7/7/26 03:28, Kefeng Wang wrote:
>>>
>>>
>>>
>>> Yes，device-private entries.
>>>
>>>
>>> I couldn't find a more suitable commit, before a3589e1d5fe3, the
>>> flush_cache_page() only called under anon_exclusive, not a device
>>> private entry, so there is no issue. This is the reason why I chose
>>> this commit.
>>
>> What makes you believe device-private entries wouldn't have PAE set?
>>
>> This likely goes back to initial support for device-private.
>>
> 
> We only need to make sure that flush_cache_page(..,pte_pfn(pte)) won't
> be called under no-present pte, I go back and find the
> flush_cache_page function was first introduced to
> migrate_vma_collect_pmd in commit 6c287605fd56, so maybe this commit is
> better for fix tag? what's your option?

That looks about right. For device-private we wouldn't need the ptep_clear_flush
(ptep_get_and_clear would be sufficient), but that's a different optimization.

-- 
Cheers,

David

Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE

[Kefeng Wang]

On 7/7/2026 9:03 PM, David Hildenbrand (Arm) wrote:
> On 7/7/26 11:33, Kefeng Wang wrote:
>>
>>
>> On 7/7/2026 3:04 PM, David Hildenbrand (Arm) wrote:
>>> On 7/7/26 03:28, Kefeng Wang wrote:
>>>>
>>>>
>>>>
>>>> Yes，device-private entries.
>>>>
>>>>
>>>> I couldn't find a more suitable commit, before a3589e1d5fe3, the
>>>> flush_cache_page() only called under anon_exclusive, not a device
>>>> private entry, so there is no issue. This is the reason why I chose
>>>> this commit.
>>>
>>> What makes you believe device-private entries wouldn't have PAE set?
>>>
>>> This likely goes back to initial support for device-private.
>>>
>>
>> We only need to make sure that flush_cache_page(..,pte_pfn(pte)) won't
>> be called under no-present pte, I go back and find the
>> flush_cache_page function was first introduced to
>> migrate_vma_collect_pmd in commit 6c287605fd56, so maybe this commit is
>> better for fix tag? what's your option?
> 
> That looks about right. For device-private we wouldn't need the ptep_clear_flush
> (ptep_get_and_clear would be sufficient), but that's a different optimization.
> 
OK， thanks for your confirm, will send v2.