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

["Mika Penttilä" <mpenttil@redhat.com>]

Hi!

On 3/5/26 01:28, Usama Arif wrote:

>
> 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@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.
>>> 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@mail.gmail.com/
>
>> Thanks! Yes, the patch makes sense
>>
>> Acked-by: Balbir Singh <balbirs@nvidia.com>
>>
>> Balbir

I remember stumbling on this while ago also. The folio_get() in migrate_vma_split_unmapped_folio()
is balanced with put_page() in __split_huge_pmd_locked() (freeze = true), can't fail for device pages.
Folios at this point are unmapped but have 1 refcount from "collecting". 
After folio_split_unmapped() the refcount(s) is still 1.

So it seems the code is good as is? A comment though would be good for the extra folio_get..

Thanks,
--Mika