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Message-ID: <64d0b302-55c5-45bb-8aa5-d00d25d8d07e@nvidia.com>
Date: Wed, 24 Sep 2025 14:47:06 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Karim Manaouil <kmanaouil.dev@...il.com>
Cc: Zi Yan <ziy@...dia.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
David Hildenbrand <david@...hat.com>, Joshua Hahn <joshua.hahnjy@...il.com>,
Rakie Kim <rakie.kim@...com>, Byungchul Park <byungchul@...com>,
Gregory Price <gourry@...rry.net>, Ying Huang
<ying.huang@...ux.alibaba.com>, Alistair Popple <apopple@...dia.com>,
Oscar Salvador <osalvador@...e.de>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
Mika Penttilä <mpenttil@...hat.com>,
Matthew Brost <matthew.brost@...el.com>,
Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v6 05/15] mm/migrate_device: handle partially mapped folios
during collection
On 9/24/25 01:56, Karim Manaouil wrote:
> On Tue, Sep 23, 2025 at 01:44:20PM +1000, Balbir Singh wrote:
>> On 9/23/25 12:23, Zi Yan wrote:
>>> On 16 Sep 2025, at 8:21, Balbir Singh wrote:
>>>
>>>> Extend migrate_vma_collect_pmd() to handle partially mapped large folios
>>>> that require splitting before migration can proceed.
>>>>
>>>> During PTE walk in the collection phase, if a large folio is only
>>>> partially mapped in the migration range, it must be split to ensure the
>>>> folio is correctly migrated.
>>>>
>>>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>>>> Cc: David Hildenbrand <david@...hat.com>
>>>> Cc: Zi Yan <ziy@...dia.com>
>>>> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
>>>> Cc: Rakie Kim <rakie.kim@...com>
>>>> Cc: Byungchul Park <byungchul@...com>
>>>> Cc: Gregory Price <gourry@...rry.net>
>>>> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
>>>> Cc: Alistair Popple <apopple@...dia.com>
>>>> Cc: Oscar Salvador <osalvador@...e.de>
>>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>>>> Cc: Nico Pache <npache@...hat.com>
>>>> Cc: Ryan Roberts <ryan.roberts@....com>
>>>> Cc: Dev Jain <dev.jain@....com>
>>>> Cc: Barry Song <baohua@...nel.org>
>>>> Cc: Lyude Paul <lyude@...hat.com>
>>>> Cc: Danilo Krummrich <dakr@...nel.org>
>>>> Cc: David Airlie <airlied@...il.com>
>>>> Cc: Simona Vetter <simona@...ll.ch>
>>>> Cc: Ralph Campbell <rcampbell@...dia.com>
>>>> Cc: Mika Penttilä <mpenttil@...hat.com>
>>>> Cc: Matthew Brost <matthew.brost@...el.com>
>>>> Cc: Francois Dugast <francois.dugast@...el.com>
>>>> ---
>>>> mm/migrate_device.c | 82 +++++++++++++++++++++++++++++++++++++++++++++
>>>> 1 file changed, 82 insertions(+)
>>>>
>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>> index abd9f6850db6..70c0601f70ea 100644
>>>> --- a/mm/migrate_device.c
>>>> +++ b/mm/migrate_device.c
>>>> @@ -54,6 +54,53 @@ static int migrate_vma_collect_hole(unsigned long start,
>>>> return 0;
>>>> }
>>>>
>>>> +/**
>>>> + * migrate_vma_split_folio() - Helper function to split a THP folio
>>>> + * @folio: the folio to split
>>>> + * @fault_page: struct page associated with the fault if any
>>>> + *
>>>> + * Returns 0 on success
>>>> + */
>>>> +static int migrate_vma_split_folio(struct folio *folio,
>>>> + struct page *fault_page)
>>>> +{
>>>> + int ret;
>>>> + struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL;
>>>> + struct folio *new_fault_folio = NULL;
>>>> +
>>>> + if (folio != fault_folio) {
>>>> + folio_get(folio);
>>>> + folio_lock(folio);
>>>> + }
>>>> +
>>>> + ret = split_folio(folio);
>>>> + if (ret) {
>>>> + if (folio != fault_folio) {
>>>> + folio_unlock(folio);
>>>> + folio_put(folio);
>>>> + }
>>>> + return ret;
>>>> + }
>>>> +
>>>> + new_fault_folio = fault_page ? page_folio(fault_page) : NULL;
>>>> +
>>>> + /*
>>>> + * Ensure the lock is held on the correct
>>>> + * folio after the split
>>>> + */
>>>> + if (!new_fault_folio) {
>>>> + folio_unlock(folio);
>>>> + folio_put(folio);
>>>> + } else if (folio != new_fault_folio) {
>>>> + folio_get(new_fault_folio);
>>>> + folio_lock(new_fault_folio);
>>>> + folio_unlock(folio);
>>>> + folio_put(folio);
>>>> + }
>>>> +
>>>> + return 0;
>>>> +}
>>>> +
>>>> static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>> unsigned long start,
>>>> unsigned long end,
>>>> @@ -136,6 +183,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>> * page table entry. Other special swap entries are not
>>>> * migratable, and we ignore regular swapped page.
>>>> */
>>>> + struct folio *folio;
>>>> +
>>>> entry = pte_to_swp_entry(pte);
>>>> if (!is_device_private_entry(entry))
>>>> goto next;
>>>> @@ -147,6 +196,23 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>> pgmap->owner != migrate->pgmap_owner)
>>>> goto next;
>>>>
>>>> + folio = page_folio(page);
>>>> + if (folio_test_large(folio)) {
>>>> + int ret;
>>>> +
>>>> + pte_unmap_unlock(ptep, ptl);
>>>> + ret = migrate_vma_split_folio(folio,
>>>> + migrate->fault_page);
>>>> +
>>>> + if (ret) {
>>>> + ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>>>> + goto next;
>>>> + }
>>>> +
>>>> + addr = start;
>>>> + goto again;
>>>> + }
>>>
>>> This does not look right to me.
>>>
>>> The folio here is device private, but migrate_vma_split_folio()
>>> calls split_folio(), which cannot handle device private folios yet.
>>> Your change to split_folio() is in Patch 10 and should be moved
>>> before this patch.
>>>
>>
>> Patch 10 is to split the folio in the middle of migration (when we have
>> converted the entries to migration entries). This patch relies on the
>> changes in patch 4. I agree the names are confusing, I'll reword the
>> functions
>
> Hi Balbir,
>
> I am still reviewing the patches, but I think I agree with Zi here.
>
> split_folio() will replace the PMD mappings of the huge folio with PTE
> mappings, but will also split the folio into smaller folios. The former
> is ok with this patch, but the latter is probably not correct if the folio
> is a zone device folio. The driver needs to know about the change, as
> usually the driver will have some sort of mapping between GPU physical
> memory chunks and their corresponding zone device pages.
>
Yes, at this point there is no support for split folio callback. I can move this
bit to a later patch or move the entire patch to after patch 10. I suspect
this is a theoretical bisection concern for a future driver using large folios?
Thanks,
Balbir
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