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Message-ID: <62fb7aca-cd30-4058-bfd1-d08f78d5c38d@nvidia.com>
Date: Tue, 23 Sep 2025 13:44:20 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Zi Yan <ziy@...dia.com>
Cc: 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 9/23/25 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
Thanks for the review
Balbir
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