Subject: Re: [RFC v2 12/21] mm: thp: handle split failure in device migration

From: Usama Arif <[email-masked]>
Date: Thu, 5 Mar 2026 19:55:22 +0300

On 02/03/2026 21:20, Nico Pache wrote:
> On Thu, Feb 26, 2026 at 4:34 AM Usama Arif <[email-masked]> wrote:
>>
>> Device memory migration has two call sites that split huge PMDs:
>>
>> migrate_vma_split_unmapped_folio():
>>   Called from migrate_vma_pages() when migrating a PMD-mapped THP to a
>>   destination that doesn't support compound pages.  It splits the PMD
>>   then splits the folio via folio_split_unmapped().
>>
>>   If the PMD split fails, folio_split_unmapped() would operate on an
>>   unsplit folio with inconsistent page table state.  Propagate -ENOMEM
>>   to skip this page's migration. This is safe as folio_split_unmapped
>>   failure would be propagated in a similar way.
>>
>> migrate_vma_insert_page():
>>   Called from migrate_vma_pages() when inserting a page into a VMA
>>   during migration back from device memory.  If a huge zero PMD exists
>>   at the target address, it must be split before PTE insertion.
>>
>>   If the split fails, the subsequent pte_alloc() and set_pte_at() would
>>   operate on a PMD slot still occupied by the huge zero entry.  Use
>>   goto abort, consistent with other allocation failures in this function.
>>
>> Signed-off-by: Usama Arif <[email-masked]>
>> ---
>>  mm/migrate_device.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 78c7acf024615..bc53e06fd9735 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -909,7 +909,13 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
>>         int ret = 0;
>>
>>         folio_get(folio);
> 
> Should we be concerned about this folio_get? Are we incrementing a
> reference that was already held if we back out of the split?
> 
> -- Nico

Hi Nico,

Thanks for pointing this out. It spun out to an entire investigation for me [1].

Similar to [1], I inserted trace prints [2] and created a new __split_huge_pmd2
that always returns -ENOMEM. Without folio_put on error [3], we get a refcount of 2.

       hmm-tests-129     [000] .l...     1.485514: __migrate_device_finalize: FINALIZE[0]: src=ffb48827440e8000 dst=ffb48827440e8000 src==dst=1 refcount_src=2 mapcount_src=0 order_src=9 migrate=0 BEFORE remove_migration_ptes
       hmm-tests-129     [000] .l...     1.485517: __migrate_device_finalize: FINALIZE[0]: src=ffb48827440e8000 refcount=3 mapcount=1 AFTER remove_migration_ptes
       hmm-tests-129     [000] .l...     1.485518: __migrate_device_finalize: FINALIZE[0]: src=ffb48827440e8000 refcount=2 AFTER folio_put(src)

With folio_put on error [4], we get a refcount of 1.

       hmm-tests-129     [001] .....     1.492216: __migrate_device_finalize: FINALIZE[0]: src=fff7b8be840f0000 dst=fff7b8be840f0000 src==dst=1 refcount_src=1 mapcount_src=0 order_src=9 migrate=0 BEFORE remove_migration_ptes
       hmm-tests-129     [001] .....     1.492219: __migrate_device_finalize: FINALIZE[0]: src=fff7b8be840f0000 refcount=2 mapcount=1 AFTER remove_migration_ptes
       hmm-tests-129     [001] .....     1.492220: __migrate_device_finalize: FINALIZE[0]: src=fff7b8be840f0000 refcount=1 AFTER folio_put(src)

So we need folio_put for split_huge_pmd_address failure, but NOT for
folio_split_unmapped.

[1] https://lore.kernel.org/all/[email-masked]/
[2] https://gist.github.com/uarif1/6abe4bedb85814e9be8d48a4fe742b41
[3] https://gist.github.com/uarif1/f718af2113bc1a33484674b61b9dafcc
[4] https://gist.github.com/uarif1/03c42f2549eaf2bc555e8b03e07a63c8