[PATCH v15 05/11] mm/hmm: migrate collection in HMM pagewalk - pte level

[mpenttil@redhat.com]

From: Mika Penttilä <mpenttil@redhat.com>

Implement the needed hmm_vma_handle_migrate_prepare() function
which is mostly carried over from migrate_device.c's
migrate_vma_collect_pmd() function.

Also implement the migrate_vma_split_folio(), for splitting
pte mapped large folios. It is also mostly from migrate_device.c,
with care taken to reference folio before relasing page table
lock.

With HMM pagewalk based migration, the idea is that
hmm_vma_handle_*() are responsible for faulting,
and the pfn collecting part. hmm_vma_handle_migrate_prepare*()
do the migration decisions (with HMM_PFN_MIGRATE), possibly split
folios, and insert migration ptes/pmds.

HMM pagewalk based migration is enabled in later commit, for now
now hmm_select_migrate() returns 0.

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>
---
 mm/hmm.c | 263 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 262 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 25f537c87b5f..a60b66963049 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -491,6 +491,63 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 #endif  /* CONFIG_ARCH_HAS_PMD_SOFTLEAVES */
 
 #ifdef CONFIG_DEVICE_MIGRATION
+/**
+ * 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
+ * @hmm_vma_walk: walk in progress
+ * @ptep: pte_t * for unmap and unlock ptl
+ *
+ * Returns 0 on success
+ */
+static int migrate_vma_split_folio(struct folio *folio,
+				   struct page *fault_page,
+				   struct hmm_vma_walk *hmm_vma_walk,
+				   pte_t *ptep)
+{
+	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);
+
+	pte_unmap_unlock(ptep, hmm_vma_walk->ptl);
+	hmm_vma_walk->ptelocked = false;
+
+	if (folio != fault_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) {
+		if (new_fault_folio != fault_folio) {
+			folio_get(new_fault_folio);
+			folio_lock(new_fault_folio);
+		}
+		folio_unlock(folio);
+		folio_put(folio);
+	}
+
+	return 0;
+}
+
 static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 					      pmd_t *pmdp,
 					      unsigned long start,
@@ -501,6 +558,11 @@ static int hmm_vma_handle_migrate_prepare_pmd(const struct mm_walk *walk,
 	return 0;
 }
 
+/*
+ * Install migration entries if migration requested, either from fault
+ * or migrate paths.
+ *
+ */
 static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
 					  pmd_t *pmdp,
 					  pte_t *ptep,
@@ -508,8 +570,207 @@ static int hmm_vma_handle_migrate_prepare(const struct mm_walk *walk,
 					  unsigned long *hmm_pfn,
 					  bool *unmapped)
 {
-	// TODO: implement migration entry insertion
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
+	struct migrate_vma *migrate = range->migrate;
+	struct mm_struct *mm = walk->vma->vm_mm;
+	struct folio *fault_folio = NULL;
+	enum migrate_vma_info minfo;
+	struct dev_pagemap *pgmap;
+	bool anon_exclusive;
+	struct folio *folio;
+	unsigned long pfn;
+	struct page *page;
+	softleaf_t entry;
+	pte_t pte, swp_pte;
+	bool writable = false;
+
+	// Do we want to migrate at all?
+	minfo = hmm_select_migrate(range);
+	if (!minfo)
+		return 0;
+
+	WARN_ON_ONCE(!migrate);
+	HMM_ASSERT_PTE_LOCKED(hmm_vma_walk, true);
+
+	fault_folio = migrate->fault_page ?
+		page_folio(migrate->fault_page) : NULL;
+
+	pte = ptep_get(ptep);
+
+	if (pte_none(pte)) {
+		if (vma_is_anonymous(walk->vma)) {
+			*hmm_pfn &= HMM_PFN_INOUT_FLAGS;
+			*hmm_pfn |= HMM_PFN_MIGRATE;
+			goto out;
+		}
+	}
+
+	if (!(hmm_pfn[0] & HMM_PFN_VALID))
+		goto out;
+
+	if (!pte_present(pte)) {
+		/*
+		 * Only care about unaddressable device page special
+		 * page table entry. Other special swap entries are not
+		 * migratable, and we ignore regular swapped page.
+		 */
+		entry = softleaf_from_pte(pte);
+		if (!softleaf_is_device_private(entry))
+			goto out;
+
+		if (!(minfo & MIGRATE_VMA_SELECT_DEVICE_PRIVATE))
+			goto out;
+
+		page = softleaf_to_page(entry);
+		folio = page_folio(page);
+		if (folio->pgmap->owner != migrate->pgmap_owner)
+			goto out;
+
+		if (folio_test_large(folio)) {
+			int ret;
+
+			ret = migrate_vma_split_folio(folio,
+						      migrate->fault_page,
+						      hmm_vma_walk,
+						      ptep);
+			if (ret)
+				goto out_error;
+			return -EAGAIN;
+		}
+
+		pfn = page_to_pfn(page);
+		if (softleaf_is_device_private_write(entry))
+			writable = true;
+	} else {
+		pfn = pte_pfn(pte);
+		if (is_zero_pfn(pfn) &&
+		    (minfo & MIGRATE_VMA_SELECT_SYSTEM)) {
+			*hmm_pfn = HMM_PFN_MIGRATE;
+			goto out;
+		}
+		page = vm_normal_page(walk->vma, addr, pte);
+		if (page && !is_zone_device_page(page) &&
+		    !(minfo & MIGRATE_VMA_SELECT_SYSTEM)) {
+			goto out;
+		} else if (page && is_device_coherent_page(page)) {
+			pgmap = page_pgmap(page);
+
+			if (!(minfo &
+			      MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
+			    pgmap->owner != migrate->pgmap_owner)
+				goto out;
+		}
+
+		folio = page ? page_folio(page) : NULL;
+		if (folio && folio_test_large(folio)) {
+			int ret;
+
+			ret = migrate_vma_split_folio(folio,
+						      migrate->fault_page,
+						      hmm_vma_walk,
+						      ptep);
+			if (ret)
+				goto out_error;
+			return -EAGAIN;
+		}
+
+		writable = pte_write(pte);
+	}
+
+	if (!page || !page->mapping)
+		goto out;
+
+	/*
+	 * By getting a reference on the folio we pin it and that blocks
+	 * any kind of migration. Side effect is that it "freezes" the
+	 * pte.
+	 *
+	 * We drop this reference after isolating the folio from the lru
+	 * for non device folio (device folio are not on the lru and thus
+	 * can't be dropped from it).
+	 */
+	folio = page_folio(page);
+	folio_get(folio);
+
+	/*
+	 * We rely on folio_trylock() to avoid deadlock between
+	 * concurrent migrations where each is waiting on the others
+	 * folio lock. If we can't immediately lock the folio we fail this
+	 * migration as it is only best effort anyway.
+	 *
+	 * If we can lock the folio it's safe to set up a migration entry
+	 * now. In the common case where the folio is mapped once in a
+	 * single process setting up the migration entry now is an
+	 * optimisation to avoid walking the rmap later with
+	 * try_to_migrate().
+	 */
+
+	if (fault_folio == folio || folio_trylock(folio)) {
+		anon_exclusive = folio_test_anon(folio) &&
+			PageAnonExclusive(page);
+
+		if (pte_present(pte))
+			flush_cache_page(walk->vma, addr, pfn);
+
+		if (anon_exclusive) {
+			pte = ptep_clear_flush(walk->vma, addr, ptep);
+
+			if (folio_try_share_anon_rmap_pte(folio, page)) {
+				set_pte_at(mm, addr, ptep, pte);
+				if (fault_folio != folio)
+					folio_unlock(folio);
+				folio_put(folio);
+				goto out;
+			}
+		} else {
+			pte = ptep_get_and_clear(mm, addr, ptep);
+		}
+
+		if (pte_present(pte) && pte_dirty(pte))
+			folio_mark_dirty(folio);
+
+		/* Setup special migration page table entry */
+		if (writable)
+			entry = make_writable_migration_entry(pfn);
+		else if (anon_exclusive)
+			entry = make_readable_exclusive_migration_entry(pfn);
+		else
+			entry = make_readable_migration_entry(pfn);
+
+		if (pte_present(pte)) {
+			if (pte_young(pte))
+				entry = make_migration_entry_young(entry);
+			if (pte_dirty(pte))
+				entry = make_migration_entry_dirty(entry);
+		}
+
+		swp_pte = swp_entry_to_pte(entry);
+		if (pte_present(pte)) {
+			if (pte_soft_dirty(pte))
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			if (pte_uffd(pte))
+				swp_pte = pte_swp_mkuffd(swp_pte);
+		} else {
+			if (pte_swp_soft_dirty(pte))
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			if (pte_swp_uffd(pte))
+				swp_pte = pte_swp_mkuffd(swp_pte);
+		}
+
+		set_pte_at(mm, addr, ptep, swp_pte);
+		folio_remove_rmap_pte(folio, page, walk->vma);
+		folio_put(folio);
+		*hmm_pfn |= HMM_PFN_MIGRATE;
+		if (pte_present(pte))
+			*unmapped = true;
+	} else {
+		folio_put(folio);
+	}
+out:
 	return 0;
+out_error:
+	return -EFAULT;
 }
 
 #else
-- 
2.55.0