Re: [PATCH] mm/migrate_device: fix folio refcount leak on folio_split_unmapped failure

[Balbir Singh <balbirs@nvidia.com>]

On 3/5/26 02:17, Zi Yan wrote:
> On 4 Mar 2026, at 7:01, Usama Arif wrote:
> 
>> From: Usama Arif <usama.arif@linux.dev>
>>
>> migrate_vma_split_unmapped_folio() takes an extra reference via
>> folio_get() before calling folio_split_unmapped().  On success, the
>> split consumes this reference: __folio_freeze_and_split_unmapped()
>> expects the +1 in its folio_ref_freeze() check, and distributes it
>> across the resulting sub-folios via folio_ref_unfreeze(...+1), which
>> are later balanced by folio_put() calls in __migrate_device_finalize().
>>
>> If folio_split_unmapped() fails (e.g., unexpected pinning returns
>> -EAGAIN), the function returns without calling folio_put().  The extra
>> reference is never released.
>>
>> Add the missing folio_put() on the error path.
>>
>> Fixes: 4265d67e405a4 ("mm/migrate_device: add THP splitting during migration")
>> Closes: https://lore.kernel.org/all/CAA1CXcDyqPPwf_-W7B+PFQtL8HdoJGCEqVsVxq7DhOUB=L4PQA@mail.gmail.com/
>> Reported-by: Nico Pache <npache@redhat.com>
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  mm/migrate_device.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 0a8b31939640f..351ecd9065d13 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -917,8 +917,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
>>  	folio_get(folio);
>>  	split_huge_pmd_address(migrate->vma, addr, true);
>>  	ret = folio_split_unmapped(folio, 0);
>> -	if (ret)
>> +	if (ret) {
>> +		folio_put(folio);
>>  		return ret;
>> +	}
>>  	migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
>>  	flags = migrate->src[idx] & ((1UL << MIGRATE_PFN_SHIFT) - 1);
>>  	pfn = migrate->src[idx] >> MIGRATE_PFN_SHIFT;
>> -- 
>> 2.47.3
> 
> Add Balbir, who wrote the code, to comment on this.
> 

Thanks Zi!

Just wondering if there is a reproducer for the issue and how the fix was tested?
I expect migrate_vma_finalize() to be called for folios, even when split failed and
drop the lock.

Balbir