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

[Balbir Singh]

On 3/5/26 08:54, Zi Yan wrote:
> 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?
> 

We reset the MIGRATE_PFN_MIGRATE flag for failing to migrate pfns.  We do a folio_put
on the src in finalize, if it is split then on all the split folios as well.

> 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?
> 

Thanks! Yes, the patch makes sense

Acked-by: Balbir Singh <balbirs@xxxxxxxxxx>

Balbir