Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Aug 11, 2025 at 10:54:04PM -0700, Matthew Brost wrote:
> On Tue, Aug 12, 2025 at 08:35:49AM +0300, Mika Penttilä wrote:
> > 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@linux-foundation.org>
> > > Cc: David Hildenbrand <david@redhat.com>
> > > Cc: Zi Yan <ziy@nvidia.com>
> > > Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> > > Cc: Rakie Kim <rakie.kim@sk.com>
> > > Cc: Byungchul Park <byungchul@sk.com>
> > > Cc: Gregory Price <gourry@gourry.net>
> > > Cc: Ying Huang <ying.huang@linux.alibaba.com>
> > > Cc: Alistair Popple <apopple@nvidia.com>
> > > Cc: Oscar Salvador <osalvador@suse.de>
> > > Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > > Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> > > Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> > > Cc: Nico Pache <npache@redhat.com>
> > > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > > Cc: Dev Jain <dev.jain@arm.com>
> > > Cc: Barry Song <baohua@kernel.org>
> > > Cc: Lyude Paul <lyude@redhat.com>
> > > Cc: Danilo Krummrich <dakr@kernel.org>
> > > Cc: David Airlie <airlied@gmail.com>
> > > Cc: Simona Vetter <simona@ffwll.ch>
> > > Cc: Ralph Campbell <rcampbell@nvidia.com>
> > > Cc: Mika Penttilä <mpenttil@redhat.com>
> > > Cc: Matthew Brost <matthew.brost@intel.com>
> > > Cc: Francois Dugast <francois.dugast@intel.com>
> > >
> > > Signed-off-by: Balbir Singh <balbirs@nvidia.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?

I also agree that, eventually, what I detail below (collecting mTHP or
THPs in PTEs) should be supported without a split, albeit enabled by a
different migrate_vma_direction flag than introduced in this series.

Reasoning for a differnet flag: Handling of mTHP/THP at the upper layers
(drivers) will have to adjusted if the THP doesn't align to a PMD.
Fairly mirror but upper layer should maintain control if they want
non-aligned mTHP/THP to PMDs collected.

I also just noticed an interface problem — migrate_vma does not define
flags as migrate_vma_direction, but I digress.

Matt

> > 
> 
> Currently, mTHP should be split upon collection, right? The only way a
> THP should be collected is if it directly maps to a PMD. If a THP or
> mTHP is found in PTEs (i.e., in migrate_vma_collect_pmd outside of
> migrate_vma_collect_huge_pmd), it should be split there. I sent this
> logic to Balbir privately, but it appears to have been omitted.
> 
> I’m quite sure this missing split is actually an upstream bug, but it
> has been suppressed by PMDs being split upon device fault. I have a test
> that performs a ton of complete mremap—nonsense no one would normally
> do, but which should work—that exposed this. I can rebase on this series
> and see if the bug appears, or try the same nonsense without the device
> faulting first and splitting the pages, to trigger the bug.
> 
> Matt
> 
> > --Mika
> >