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Message-ID: <3FE3F2B9-86F9-4EED-863A-093FE3959B1A@nvidia.com>
Date: Mon, 22 Sep 2025 22:23:34 -0400
From: Zi Yan <ziy@...dia.com>
To: Balbir Singh <balbirs@...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 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.
> +
> 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 ? page_folio(page) : NULL;
> + if (folio && 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
--
Best Regards,
Yan, Zi
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