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Message-ID: <ea6caec5-fd20-444c-b937-6cab61198c46@redhat.com>
Date: Wed, 3 Sep 2025 07:40:42 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Balbir Singh <balbirs@...dia.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: [v4 05/15] mm/migrate_device: handle partially mapped folios
during collection
Hi,
On 9/3/25 04:18, 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 | 95 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 95 insertions(+)
>
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index e05e14d6eacd..e58c3f9d01c8 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -54,6 +54,54 @@ static int migrate_vma_collect_hole(unsigned long start,
> return 0;
> }
>
> +/**
> + * migrate_vma_split_folio - Helper function to split a(n) (m)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 +184,8 @@ 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 +197,29 @@ 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 +244,28 @@ 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
> + */
There are other reasons like vma splits for various reasons.
> + 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;
> }
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