Re: [PATCH] mm/migrate_device: fix double unlock and remove dead code

[David Hildenbrand (Arm)]

On 4/14/26 01:30, Matthew Brost wrote:
> On Mon, Apr 13, 2026 at 06:39:23PM +0530, Sunny Patel wrote:
>> Fix two bugs in device migration paths:
>>
>> 1) migrate_vma_collect_huge_pmd() calls spin_unlock after
>>    softleaf_entry_wait_on_locked(), which already releases the ptl.
>>
>> 2) migrate_vma_insert_huge_pmd_page() has a dead else-if branch and this
>>    branch is always unreachable.
>>
>> Signed-off-by: Sunny Patel <nueralspacetech@xxxxxxxxx>
>> ---
>>  mm/migrate_device.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 8079676c8f1f..0e005c26ee88 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -177,7 +177,6 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
>>  
>>  		if (softleaf_is_migration(entry)) {
>>  			softleaf_entry_wait_on_locked(entry, ptl);
>> -			spin_unlock(ptl);
>>  			return -EAGAIN;
>>  		}
> 
> This entire if statement is dead code, since we bail out and collect a
> skip on !softleaf_is_device_private immediately above. I thought we
> already had a patch from Davidlohr Bueso 'mm/migrate_device: remove dead
> migration entry check in migrate_vma_collect_huge_pmd()' that removed
> this whole if statement. I even have an email from Andrew pulling this
> patch, but it looks like the code is still present in 7.0.
> 
> I found this link via a google search which contains the above patch
> too:
> 
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/akpm/25-new/+/f405e7bc5d93a541cf11663b2424baf0a03b496f%5E%21/
> 
> Look like this change got lost somehow?

Oh, even better, then we don't even need a Fixes:

-- 
Cheers,

David