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

[Usama Arif]

On 04/03/2026 22:09, Balbir Singh wrote:
> 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?
>>

Hello!

Thanks for reviewing everyone. So its very difficult to create a reproducer I think
the extra reference would need to appear after migrate_device_unmap() but before
folio_split_unmapped() in migrate_vma_pages()? That's hard to trigger reliably from
userspace.

The fix came about when Nico indicated there might be an issue if split_huge_pmd_address
fails in my patch [1].

Below is my understanding of how refcounting is working over here step by step. I
might very well be wrong on this, and the refcounting is a bit all over the place
and I might miss a reference change somewhere so would really appreciate if someone
can confirm this!


1. migrate_vma_collect_huge_pmd():
  a) folio_get(folio) -> +1 (collect reference)
2. migrate_device_unmap():
  a) folio_isolate_lru() -> +1 (isolation reference)
  b) folio_put() -> -1 (drops the collect reference)


Without this patch fix:

3. migrate_vma_split_unmapped_folio():
  a) folio_get(folio) -> +1 (split reference)
  b) folio_split_unmapped() -> fails
  c) Returns error — without folio_put() which is the fix
4. Caller in migrate_vma_pages(): clears MIGRATE_PFN_MIGRATE | MIGRATE_PFN_COMPOUND
5. __migrate_device_finalize(): sees !(src_pfns[i] & MIGRATE_PFN_MIGRATE), restores the folio:
  a) remove_migration_ptes(src, src) — re-establishes user PTEs
  b) folio_unlock(src)
  c) folio_put(src) -> -1 (drops the isolation reference)

The split reference in 3.a is never released and the folio has a permanently elevated refcount.
Unless I missed a folio_put somewhere for the refcount increase in folio_isolate_lru() (2.b)?

Please let me know if this makes sense!

[1] https://lore.kernel.org/all/CAA1CXcDyqPPwf_-W7B+PFQtL8HdoJGCEqVsVxq7DhOUB=L4PQA@xxxxxxxxxxxxxx/

> 
> Thanks! Yes, the patch makes sense
> 
> Acked-by: Balbir Singh <balbirs@xxxxxxxxxx>
> 
> Balbir