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Message-ID: <388ddd8a-c71c-41f1-8706-12ebeea0fc7b@nvidia.com>
Date: Tue, 16 Sep 2025 20:50:45 +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 06/15] mm/migrate_device: implement THP migration of zone
device pages
On 9/12/25 15:38, Mika Penttilä wrote:
>
> On 9/12/25 08:28, Mika Penttilä wrote:
>
>> On 9/12/25 08:04, Balbir Singh wrote:
>>
>>> On 9/11/25 21:52, Mika Penttilä wrote:
>>>> sending again for the v5 thread..
>>>>
>>>> On 9/8/25 03:04, Balbir Singh wrote:
>>>>
>>>>> MIGRATE_VMA_SELECT_COMPOUND will be used to select THP pages during
>>>>> migrate_vma_setup() and MIGRATE_PFN_COMPOUND will make migrating
>>>>> device pages as compound pages during device pfn migration.
>>>>>
>>>>> migrate_device code paths go through the collect, setup
>>>>> and finalize phases of migration.
>>>>>
>>>>> The entries in src and dst arrays passed to these functions still
>>>>> remain at a PAGE_SIZE granularity. When a compound page is passed,
>>>>> the first entry has the PFN along with MIGRATE_PFN_COMPOUND
>>>>> and other flags set (MIGRATE_PFN_MIGRATE, MIGRATE_PFN_VALID), the
>>>>> remaining entries (HPAGE_PMD_NR - 1) are filled with 0's. This
>>>>> representation allows for the compound page to be split into smaller
>>>>> page sizes.
>>>>>
>>>>> migrate_vma_collect_hole(), migrate_vma_collect_pmd() are now THP
>>>>> page aware. Two new helper functions migrate_vma_collect_huge_pmd()
>>>>> and migrate_vma_insert_huge_pmd_page() have been added.
>>>>>
>>>>> migrate_vma_collect_huge_pmd() can collect THP pages, but if for
>>>>> some reason this fails, there is fallback support to split the folio
>>>>> and migrate it.
>>>>>
>>>>> migrate_vma_insert_huge_pmd_page() closely follows the logic of
>>>>> migrate_vma_insert_page()
>>>>>
>>>>> Support for splitting pages as needed for migration will follow in
>>>>> later patches in this series.
>>>>>
>>>>> 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>
>>>>> ---
>>>>> include/linux/migrate.h | 2 +
>>>>> mm/migrate_device.c | 456 ++++++++++++++++++++++++++++++++++------
>>>>> 2 files changed, 395 insertions(+), 63 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>>>>> index 1f0ac122c3bf..41b4cc05a450 100644
>>>>> --- a/include/linux/migrate.h
>>>>> +++ b/include/linux/migrate.h
>>>>> @@ -125,6 +125,7 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
>>>>> #define MIGRATE_PFN_VALID (1UL << 0)
>>>>> #define MIGRATE_PFN_MIGRATE (1UL << 1)
>>>>> #define MIGRATE_PFN_WRITE (1UL << 3)
>>>>> +#define MIGRATE_PFN_COMPOUND (1UL << 4)
>>>>> #define MIGRATE_PFN_SHIFT 6
>>>>>
>>>>> static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
>>>>> @@ -143,6 +144,7 @@ enum migrate_vma_direction {
>>>>> MIGRATE_VMA_SELECT_SYSTEM = 1 << 0,
>>>>> MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
>>>>> MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,
>>>>> + MIGRATE_VMA_SELECT_COMPOUND = 1 << 3,
>>>>> };
>>>>>
>>>>> struct migrate_vma {
>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>>> index f45ef182287d..1dfcf4799ea5 100644
>>>>> --- a/mm/migrate_device.c
>>>>> +++ b/mm/migrate_device.c
>>>>> @@ -14,6 +14,7 @@
>>>>> #include <linux/pagewalk.h>
>>>>> #include <linux/rmap.h>
>>>>> #include <linux/swapops.h>
>>>>> +#include <linux/pgalloc.h>
>>>>> #include <asm/tlbflush.h>
>>>>> #include "internal.h"
>>>>>
>>>>> @@ -44,6 +45,23 @@ static int migrate_vma_collect_hole(unsigned long start,
>>>>> if (!vma_is_anonymous(walk->vma))
>>>>> return migrate_vma_collect_skip(start, end, walk);
>>>>>
>>>>> + if (thp_migration_supported() &&
>>>>> + (migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
>>>>> + (IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
>>>>> + IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
>>>>> + migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE |
>>>>> + MIGRATE_PFN_COMPOUND;
>>>>> + migrate->dst[migrate->npages] = 0;
>>>>> + migrate->npages++;
>>>>> + migrate->cpages++;
>>>>> +
>>>>> + /*
>>>>> + * Collect the remaining entries as holes, in case we
>>>>> + * need to split later
>>>>> + */
>>>>> + return migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
>>>>> + }
>>>>> +
>>>> seems you have to split_huge_pmd() for the huge zero page here in case
>>>> of !thp_migration_supported() afaics
>>>>
>>> Not really, if pfn is 0, we do a vm_insert_page (please see if (!page) line 1107) and
>>> folio handling in migrate_vma_finalize line 1284
>> Ok actually seems it is handled by migrate_vma_insert_page() which does
>>
>> if (!pmd_none(*pmdp)) {
>> if (pmd_trans_huge(*pmdp)) {
>> if (!is_huge_zero_pmd(*pmdp))
>> goto abort;
>> folio_get(pmd_folio(*pmdp));
>> split_huge_pmd(vma, pmdp, addr); <----- here
>> } else if (pmd_leaf(*pmdp))
>> goto abort;
>> }
>>
> While at it, think the folio_get(pmd_folio(*pmdp)); is wrong for here,
> we split the pmd for huge zero page.
>
Ack, will do
Thanks for the review
Balbir
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