Re: [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/26/26 12:42, Lorenzo Stoakes wrote:
> On Fri, Jun 26, 2026 at 12:07:56PM +0200, David Hildenbrand (Arm) wrote:
>> On 6/24/26 08:53, Wei Yang wrote:
>>> Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
>>> device-private entries") introduced the concept of device-private
>>> PMD entries, but did not correctly update the rmap walk code to
>>> account for them.
>>>
>>> As a result, when page_vma_mapped_walk() encounters device-private
>>> PMD entries, it takes no action other than to acquire the PMD lock
>>> and exit.
>>>
>>> However this is highly problematic for two reasons - firstly,
>>> device private entries possess a PFN so check_pmd() needs to be
>>> called to ensure an overlapping PFN range.
>>>
>>> Secondly, and more importantly, if PVMW_MIGRATION is set the
>>> caller assumes the returned entry is a migration entry, resulting
>>> in memory corruption when the caller tries to interpret the device
>>> private entry as such.
>>>
>>> In addition, commit 146287290023 ("mm/huge_memory: implement
>>> device-private THP splitting") allowed device private PMDs to be
>>> split like THP mappings, but again did not update this code path.
>>>
>>> As a result, we might race a PMD split prior to acquiring the PMD
>>> lock.
>>>
>>> This patch addresses all of these issues by invoking check_pmd(),
>>> ensuring PMVW_MIGRATION is not set and checks whether a split raced
>>> us we do for PMD THP and migration entries.
>>>
>>> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>> Suggested-by: David Hildenbrand <david@kernel.org>
>>> Cc: David Hildenbrand <david@kernel.org>
>>> Cc: Balbir Singh <balbirs@nvidia.com>
>>> Cc: SeongJae Park <sj@kernel.org>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Lorenzo Stoakes <ljs@kernel.org>
>>> Cc: Lance Yang <lance.yang@linux.dev>
>>>
>>> ---
>>> v4:
>>>   * refine subject and commit log based on Lorenzo's suggestion
>>>   * put pmd device-private entry handling in its own if branch,
>>>     suggested by Lorenzo
>>>
>>> v3:
>>>   * remove cleanup part, only fix the issue for device-private entry
>>>   * refine user effect description based on Lorenzo's suggestion
>>>
>>> v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u
>>>   * specify the possible error case of current code and user visible effect
>>>   * besides fix, cleanup the pmd entry handling based on David's suggestion
>>>
>>> v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/
>>> ---
>>>  mm/page_vma_mapped.c | 20 +++++++++++++++-----
>>>  1 file changed, 15 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index 2ccbabfb2cc1..17dff8aab9f9 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -269,14 +269,24 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>  			/* THP pmd was split under us: handle on pte level */
>>>  			spin_unlock(pvmw->ptl);
>>>  			pvmw->ptl = NULL;
>>> -		} else if (!pmd_present(pmde)) {
>>> -			const softleaf_t entry = softleaf_from_pmd(pmde);
>>> +		} else if (pmd_is_device_private_entry(pmde)) {
>>> +			softleaf_t entry;
>>> +
>>> +			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>> +			pmde = *pvmw->pmd;
>>> +			entry = softleaf_from_pmd(pmde);
>>>
>>> -			if (softleaf_is_device_private(entry)) {
>>> -				pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>> +			if (likely(softleaf_is_device_private(entry))) {
>>> +				if (pvmw->flags & PVMW_MIGRATION)
>>> +					return not_found(pvmw);
>>> +				if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>> +					return not_found(pvmw);
>>>  				return true;
>>>  			}
>>> -
>>> +			/* device-private pmd was split under us: handle on pte level */
>>> +			spin_unlock(pvmw->ptl);
>>> +			pvmw->ptl = NULL;
>>> +		} else if (!pmd_present(pmde)) {
>>>  			if ((pvmw->flags & PVMW_SYNC) &&
>>>  			    thp_vma_suitable_order(vma, pvmw->address,
>>>  						   PMD_ORDER) &&
>>
>> This is extremely hard to review given the existing crap handling here. I'm
>> really sorry, but it makes my head hurt (I'm not kidding :) ).
>>
>> It's completely unclear why we only have to check for a subset of the cases
>> after taking the lock.
>>
>> Could we simply extend the existing migration pmd handling and leave the
>> !pmd_present() case for pmd_none()?
>>
>> That leaves no question to "which transitions are actually allowed", including
>> "could we accidentally assume something is a page table when really it isn't".
>>
>>
>> So what about something like the following?
>>
>> The "thp_migration_supported()" is not required when checking for
>> pmd_is_migration_entry(), as that defaults to "false" when not compiled in.
>>
>> Untested:
>>
>>
>> From 048ecd33673ec649e168fbbb97749a7c0e344fcd Mon Sep 17 00:00:00 2001
>> From: "David Hildenbrand (Arm)" <david@kernel.org>
>> Date: Fri, 26 Jun 2026 12:03:40 +0200
>> Subject: [PATCH] tmp
>>
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>> ---
>>  mm/page_vma_mapped.c | 29 +++++++++++++++++------------
>>  1 file changed, 17 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index 2ccbabfb2cc17..ed2a23a90e8dd 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -243,21 +243,31 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>  		 */
>>  		pmde = pmdp_get_lockless(pvmw->pmd);
>>
>> -		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
>> +		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
>> +		    pmd_is_device_private_entry(pmde)) {
>>  			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>  			pmde = *pvmw->pmd;
>> -			if (!pmd_present(pmde)) {
>> +			if (pmd_is_migration_entry(pmde)) {
>>  				softleaf_t entry;
>>
>> -				if (!thp_migration_supported() ||
> 
> Do we care about this? Or is !tmp_migration_supported() -> implies you
> wouldn't see a migration entry here anyway?

Yeah, I noted above

"The "thp_migration_supported()" is not required when checking for
pmd_is_migration_entry(), as that defaults to "false" when not compiled in."

Given that

tmp_migration_supported() -> IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);$

And

pmd_is_migration_entry() -> softleaf_is_migration(softleaf_from_pmd(pmd));

whereby softleaf_from_pmd() only returns something non-none for
CONFIG_ARCH_ENABLE_THP_MIGRATION.

> 
> Maybe worth a VM_WARN_ON_ONCE()?

I think it was primarily a a hack to slightly optimize code generated for
!CONFIG_ARCH_ENABLE_THP_MIGRATION, not really something for correctness as it seems.

So I think we can safely drop it. :)

-- 
Cheers,

David