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Message-ID: <81ca37d5-b1ff-46de-8dcc-b222af350c77@redhat.com>
Date: Tue, 12 Aug 2025 08:35:49 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, dri-devel@...ts.freedesktop.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: 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: [v3 03/11] mm/migrate_device: THP migration of zone device pages
Hi,
On 8/12/25 05:40, 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 | 457 ++++++++++++++++++++++++++++++++++------
> 2 files changed, 396 insertions(+), 63 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index acadd41e0b5c..d9cef0819f91 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -129,6 +129,7 @@ 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)
> @@ -147,6 +148,7 @@ 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 0ed337f94fcd..6621bba62710 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 <asm/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);
> + }
> +
> for (addr = start; addr < end; addr += PAGE_SIZE) {
> migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
> migrate->dst[migrate->npages] = 0;
> @@ -54,57 +72,151 @@ static int migrate_vma_collect_hole(unsigned long start,
> return 0;
> }
>
> -static int migrate_vma_collect_pmd(pmd_t *pmdp,
> - unsigned long start,
> - unsigned long end,
> - struct mm_walk *walk)
> +/**
> + * migrate_vma_collect_huge_pmd - collect THP pages without splitting the
> + * folio for device private pages.
> + * @pmdp: pointer to pmd entry
> + * @start: start address of the range for migration
> + * @end: end address of the range for migration
> + * @walk: mm_walk callback structure
> + *
> + * Collect the huge pmd entry at @pmdp for migration and set the
> + * MIGRATE_PFN_COMPOUND flag in the migrate src entry to indicate that
> + * migration will occur at HPAGE_PMD granularity
> + */
> +static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
> + unsigned long end, struct mm_walk *walk,
> + struct folio *fault_folio)
> {
> + struct mm_struct *mm = walk->mm;
> + struct folio *folio;
> struct migrate_vma *migrate = walk->private;
> - struct folio *fault_folio = migrate->fault_page ?
> - page_folio(migrate->fault_page) : NULL;
> - struct vm_area_struct *vma = walk->vma;
> - struct mm_struct *mm = vma->vm_mm;
> - unsigned long addr = start, unmapped = 0;
> spinlock_t *ptl;
> - pte_t *ptep;
> + swp_entry_t entry;
> + int ret;
> + unsigned long write = 0;
>
> -again:
> - if (pmd_none(*pmdp))
> + ptl = pmd_lock(mm, pmdp);
> + if (pmd_none(*pmdp)) {
> + spin_unlock(ptl);
> return migrate_vma_collect_hole(start, end, -1, walk);
> + }
>
> if (pmd_trans_huge(*pmdp)) {
> - struct folio *folio;
> -
> - ptl = pmd_lock(mm, pmdp);
> - if (unlikely(!pmd_trans_huge(*pmdp))) {
> + if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
> spin_unlock(ptl);
> - goto again;
> + return migrate_vma_collect_skip(start, end, walk);
> }
>
> folio = pmd_folio(*pmdp);
> if (is_huge_zero_folio(folio)) {
> spin_unlock(ptl);
> - split_huge_pmd(vma, pmdp, addr);
> - } else {
> - int ret;
> + return migrate_vma_collect_hole(start, end, -1, walk);
> + }
> + if (pmd_write(*pmdp))
> + write = MIGRATE_PFN_WRITE;
> + } else if (!pmd_present(*pmdp)) {
> + entry = pmd_to_swp_entry(*pmdp);
> + folio = pfn_swap_entry_folio(entry);
> +
> + if (!is_device_private_entry(entry) ||
> + !(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
> + (folio->pgmap->owner != migrate->pgmap_owner)) {
> + spin_unlock(ptl);
> + return migrate_vma_collect_skip(start, end, walk);
> + }
>
> - folio_get(folio);
> + if (is_migration_entry(entry)) {
> + migration_entry_wait_on_locked(entry, ptl);
> spin_unlock(ptl);
> - /* FIXME: we don't expect THP for fault_folio */
> - if (WARN_ON_ONCE(fault_folio == folio))
> - return migrate_vma_collect_skip(start, end,
> - walk);
> - if (unlikely(!folio_trylock(folio)))
> - return migrate_vma_collect_skip(start, end,
> - walk);
> - ret = split_folio(folio);
> - if (fault_folio != folio)
> - folio_unlock(folio);
> - folio_put(folio);
> - if (ret)
> - return migrate_vma_collect_skip(start, end,
> - walk);
> + return -EAGAIN;
> }
> +
> + if (is_writable_device_private_entry(entry))
> + write = MIGRATE_PFN_WRITE;
> + } else {
> + spin_unlock(ptl);
> + return -EAGAIN;
> + }
> +
> + folio_get(folio);
> + if (folio != fault_folio && unlikely(!folio_trylock(folio))) {
> + spin_unlock(ptl);
> + folio_put(folio);
> + 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))) {
> +
> + struct page_vma_mapped_walk pvmw = {
> + .ptl = ptl,
> + .address = start,
> + .pmd = pmdp,
> + .vma = walk->vma,
> + };
> +
> + unsigned long pfn = page_to_pfn(folio_page(folio, 0));
> +
> + migrate->src[migrate->npages] = migrate_pfn(pfn) | write
> + | MIGRATE_PFN_MIGRATE
> + | MIGRATE_PFN_COMPOUND;
> + migrate->dst[migrate->npages++] = 0;
> + migrate->cpages++;
> + ret = set_pmd_migration_entry(&pvmw, folio_page(folio, 0));
> + if (ret) {
> + migrate->npages--;
> + migrate->cpages--;
> + migrate->src[migrate->npages] = 0;
> + migrate->dst[migrate->npages] = 0;
> + goto fallback;
> + }
> + migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
> + spin_unlock(ptl);
> + return 0;
> + }
> +
> +fallback:
> + spin_unlock(ptl);
> + if (!folio_test_large(folio))
> + goto done;
> + ret = split_folio(folio);
> + if (fault_folio != folio)
> + folio_unlock(folio);
> + folio_put(folio);
> + if (ret)
> + return migrate_vma_collect_skip(start, end, walk);
> + if (pmd_none(pmdp_get_lockless(pmdp)))
> + return migrate_vma_collect_hole(start, end, -1, walk);
> +
> +done:
> + return -ENOENT;
> +}
> +
> +static int migrate_vma_collect_pmd(pmd_t *pmdp,
> + unsigned long start,
> + unsigned long end,
> + struct mm_walk *walk)
> +{
> + struct migrate_vma *migrate = walk->private;
> + struct vm_area_struct *vma = walk->vma;
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long addr = start, unmapped = 0;
> + spinlock_t *ptl;
> + struct folio *fault_folio = migrate->fault_page ?
> + page_folio(migrate->fault_page) : NULL;
> + pte_t *ptep;
> +
> +again:
> + if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) {
> + int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio);
> +
> + if (ret == -EAGAIN)
> + goto again;
> + if (ret == 0)
> + return 0;
> }
>
> ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> @@ -222,8 +334,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
> }
>
> - /* FIXME support THP */
> - if (!page || !page->mapping || PageTransCompound(page)) {
> + if (!page || !page->mapping) {
> mpfn = 0;
> goto next;
> }
> @@ -394,14 +505,6 @@ static bool migrate_vma_check_page(struct page *page, struct page *fault_page)
> */
> int extra = 1 + (page == fault_page);
>
> - /*
> - * FIXME support THP (transparent huge page), it is bit more complex to
> - * check them than regular pages, because they can be mapped with a pmd
> - * or with a pte (split pte mapping).
> - */
> - if (folio_test_large(folio))
> - return false;
> -
You cannot remove this check unless support normal mTHP folios migrate to device,
which I think this series doesn't do, but maybe should?
--Mika
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