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Message-ID: <8e1ef390-ab28-4294-8528-c57453e3acc1@nvidia.com>
Date: Mon, 8 Sep 2025 14:57:23 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Mika Penttilä <mpenttil@...hat.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.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>,
Matthew Brost <matthew.brost@...el.com>,
Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v5 05/15] mm/migrate_device: handle partially mapped folios
during collection
On 9/8/25 14:14, Mika Penttilä wrote:
> Hi,
>
> On 9/8/25 03:04, 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.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> 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>
>>
>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>> ---
>> mm/migrate_device.c | 94 +++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 94 insertions(+)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index abd9f6850db6..f45ef182287d 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 +196,29 @@ 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;
>> +
>> + /*
>> + * The reason for finding pmd present with a
>> + * large folio for the pte is partial unmaps.
>> + * Split the folio now for the migration to be
>> + * handled correctly
>> + */
>> + 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;
>> + }
>> +
>> mpfn = migrate_pfn(page_to_pfn(page)) |
>> MIGRATE_PFN_MIGRATE;
>> if (is_writable_device_private_entry(entry))
>> @@ -171,6 +243,28 @@ 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;
>> +
>> + /*
>> + * The reason for finding pmd present with a
>> + * large folio for the pte is partial unmaps.
>> + * Split the folio now for the migration to be
>> + * handled correctly
>> + */
>
> This comment is still not changed, there are other reasons for pte mapped large pages.
> Also now all the mTHPs are splitted, which is change of behavior (currently ignored)
> for order < PMD_ORDER.
Oh! sorry I missed it. I am attaching the version with the comments removed.
On the behaviour change, I agree, but it is required for migration to occur.
Updated patch below:
mm/migrate_device: handle partially mapped folios during collection
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.
Cc: Andrew Morton <akpm@...ux-foundation.org>
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>
Signed-off-by: Balbir Singh <balbirs@...dia.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..0afdc8b67c60 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;
+ }
+
mpfn = migrate_pfn(page_to_pfn(page)) |
MIGRATE_PFN_MIGRATE;
if (is_writable_device_private_entry(entry))
@@ -171,6 +237,22 @@ 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;
+ }
mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
}
--
2.50.1
Balbir Singh
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