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

[Zi Yan]

On 4 Mar 2026, at 16:48, Balbir Singh wrote:

> On 3/5/26 02:17, Zi Yan wrote:
>> On 4 Mar 2026, at 7:01, Usama Arif wrote:
>>
>>> From: Usama Arif <usama.arif@xxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxxxxx/
>>> Reported-by: Nico Pache <npache@xxxxxxxxxx>
>>> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
>>> ---
>>>  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.

Does migrate_vma_finalize() do folio_put() for failed-to-split folios?
If so, how does it distinguish between split folios and failed-to-split folios?
By comparing source and destination folio orders?

What we see from migrate_vma_split_unmapped_folio() is that
it adds a refcount for all input folios, but only drops a refcount
for the split folio. Isn’t it cause failed-to-split folios to have
additional refcount?


Best Regards,
Yan, Zi