[PATCH v7 2/6] mm: Add helper to convert HMM pfn to migrate pfn

mpenttil
2026-03-30T04:30:48Z

The unified HMM/migrate_device pagewalk does the "collecting"
on the HMM side, so we need a helper to transfer pfns to the
migrate_vma world.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
---
include/linux/hmm.h | 18 ++++++++++++-
include/linux/migrate.h | 3 ++-
mm/migrate_device.c | 57 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 76 insertions(+), 2 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index db75ffc949a7..9adc22b73533 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -13,6 +13,8 @@

struct mmu_interval_notifier;

+struct migrate_vma;
+
/*
* On output:
* 0 - The page is faultable and a future call with
@@ -27,6 +29,12 @@ struct mmu_interval_notifier;
* HMM_PFN_P2PDMA_BUS - Bus mapped P2P transfer
* HMM_PFN_DMA_MAPPED - Flag preserved on input-to-output transformation
* to mark that page is already DMA mapped
+ * HMM_PFN_MIGRATE - The entry is to be migrated. Note, HMM_PFN_MIGRATE
+ * alone without HMM_PFN_VALID denotes the
+ * empty page.
+ * This flag together with HMM_PFN_COMPOUND are
+ * indicators for migrate_hmm_range_setup() to
+ * setup the migrate pfns.
*
* On input:
* 0 - Return the current state of the page, do not fault it.
@@ -34,6 +42,8 @@ struct mmu_interval_notifier;
* will fail
* HMM_PFN_REQ_WRITE - The output must have HMM_PFN_WRITE or hmm_range_fault()
* will fail. Must be combined with HMM_PFN_REQ_FAULT.
+ * HMM_PFN_REQ_MIGRATE - For default_flags, request to migrate, according to
+ * hmm_range.migrate.flags
*/
enum hmm_pfn_flags {
/* Output fields and flags */
@@ -48,11 +58,15 @@ enum hmm_pfn_flags {
HMM_PFN_P2PDMA = 1UL << (BITS_PER_LONG - 5),
HMM_PFN_P2PDMA_BUS = 1UL << (BITS_PER_LONG - 6),

- HMM_PFN_ORDER_SHIFT = (BITS_PER_LONG - 11),
+ /* Migrate request */
+ HMM_PFN_MIGRATE = 1UL << (BITS_PER_LONG - 7),
+ HMM_PFN_COMPOUND = 1UL << (BITS_PER_LONG - 8),
+ HMM_PFN_ORDER_SHIFT = (BITS_PER_LONG - 13),

/* Input flags */
HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
+ HMM_PFN_REQ_MIGRATE = HMM_PFN_MIGRATE,

HMM_PFN_FLAGS = ~((1UL << HMM_PFN_ORDER_SHIFT) - 1),
};
@@ -107,6 +121,7 @@ static inline unsigned int hmm_pfn_to_map_order(unsigned long hmm_pfn)
* @default_flags: default flags for the range (write, read, ... see hmm doc)
* @pfn_flags_mask: allows to mask pfn flags so that only default_flags matter
* @dev_private_owner: owner of device private pages
+ * @migrate: structure for migrating a range of a VMA
*/
struct hmm_range {
struct mmu_interval_notifier *notifier;
@@ -117,6 +132,7 @@ struct hmm_range {
unsigned long default_flags;
unsigned long pfn_flags_mask;
void *dev_private_owner;
+ struct migrate_vma *migrate;
};

/*
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index d5af2b7f577b..425ab5242da0 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -3,6 +3,7 @@
#define _LINUX_MIGRATE_H

#include <linux/mm.h>
+#include <linux/hmm.h>
#include <linux/mempolicy.h>
#include <linux/migrate_mode.h>
#include <linux/hugetlb.h>
@@ -200,7 +201,7 @@ void migrate_device_pages(unsigned long *src_pfns, unsigned long *dst_pfns,
unsigned long npages);
void migrate_device_finalize(unsigned long *src_pfns,
unsigned long *dst_pfns, unsigned long npages);
-
+void migrate_hmm_range_setup(struct hmm_range *range);
#endif /* CONFIG_MIGRATION */

#endif /* _LINUX_MIGRATE_H */
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 8079676c8f1f..a4062fd21490 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1489,3 +1489,60 @@ int migrate_device_coherent_folio(struct folio *folio)
return 0;
return -EBUSY;
}
+
+/**
+ * migrate_hmm_range_setup() - prepare to migrate a range of memory
+ * @range: contains pointer to struct migrate_vma to be set up.
+ *
+ * When collecting has been done with hmm_range_fault(), this
+ * should be called next, and completes range->migrate by
+ * populating migrate->src[] and migrate->dst[]
+ * using range->hmm_pfns[].
+ * Also, migrate->cpages and migrate->npages get initialized.
+ * After migrate_hmm_range_setup(), range->migrate is good
+ * for the rest of the migrate_vma_* flow.
+ */
+void migrate_hmm_range_setup(struct hmm_range *range)
+{
+
+ struct migrate_vma *migrate = range->migrate;
+
+ if (!migrate)
+ return;
+
+ migrate->npages = (migrate->end - migrate->start) >> PAGE_SHIFT;
+ migrate->cpages = 0;
+
+ for (unsigned long i = 0; i < migrate->npages; i++) {
+
+ unsigned long pfn = range->hmm_pfns[i];
+
+ /*
+ * We are only interested in entries to be
+ * migrated.
+ */
+ if (!(pfn & HMM_PFN_MIGRATE)) {
+ migrate->src[i] = 0;
+ migrate->dst[i] = 0;
+ continue;
+ }
+
+ migrate->cpages++;
+
+ /* HMM_PFN_MIGRATE without HMM_PFN_VALID denotes the special zero page */
+ if (pfn & (HMM_PFN_VALID))
+ migrate->src[i] = migrate_pfn(page_to_pfn(hmm_pfn_to_page(pfn)))
+ | MIGRATE_PFN_MIGRATE;
+ else
+ migrate->src[i] = MIGRATE_PFN_MIGRATE;
+
+ migrate->src[i] |= (pfn & HMM_PFN_WRITE) ? MIGRATE_PFN_WRITE : 0;
+ migrate->src[i] |= (pfn & HMM_PFN_COMPOUND) ? MIGRATE_PFN_COMPOUND : 0;
+ migrate->dst[i] = 0;
+ }
+
+ if (migrate->cpages)
+ migrate_vma_unmap(migrate);
+
+}
+EXPORT_SYMBOL(migrate_hmm_range_setup);
--
2.50.0