[PATCH v5 4/5] drm/pagemap: Correct cpages calculation for migrate_vma_setup

Francois Dugast
2026-01-14T19:22:00Z

From: Matthew Brost <matthew.brost@intel.com>

cpages returned from migrate_vma_setup represents the total number of
individual pages found, not the number of 4K pages. The math in
drm_pagemap_migrate_to_devmem for npages is based on the number of 4K
pages, so cpages != npages can fail even if the entire memory range is
found in migrate_vma_setup (e.g., when a single 2M page is found).
Add drm_pagemap_cpages, which converts cpages to the number of 4K pages
found.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: linux-mm@kvack.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
---
drivers/gpu/drm/drm_pagemap.c | 38 ++++++++++++++++++++++++++++++++++-
1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
index f613b4d48499..3fc466f04b13 100644
--- a/drivers/gpu/drm/drm_pagemap.c
+++ b/drivers/gpu/drm/drm_pagemap.c
@@ -452,6 +452,41 @@ static int drm_pagemap_migrate_range(struct drm_pagemap_devmem *devmem,
return ret;
}

+/**
+ * drm_pagemap_cpages() - Count collected pages
+ * @migrate_pfn: Array of migrate_pfn entries to account
+ * @npages: Number of entries in @migrate_pfn
+ *
+ * Compute the total number of minimum-sized pages represented by the
+ * collected entries in @migrate_pfn. The total is derived from the
+ * order encoded in each entry.
+ *
+ * Return: Total number of minimum-sized pages.
+ */
+static int drm_pagemap_cpages(unsigned long *migrate_pfn, unsigned long npages)
+{
+ unsigned long i, cpages = 0;
+
+ for (i = 0; i < npages;) {
+ struct page *page = migrate_pfn_to_page(migrate_pfn[i]);
+ struct folio *folio;
+ unsigned int order = 0;
+
+ if (page) {
+ folio = page_folio(page);
+ order = folio_order(folio);
+ cpages += NR_PAGES(order);
+ } else if (migrate_pfn[i] & MIGRATE_PFN_COMPOUND) {
+ order = HPAGE_PMD_ORDER;
+ cpages += NR_PAGES(order);
+ }
+
+ i += NR_PAGES(order);
+ }
+
+ return cpages;
+}
+
/**
* drm_pagemap_migrate_to_devmem() - Migrate a struct mm_struct range to device memory
* @devmem_allocation: The device memory allocation to migrate to.
@@ -564,7 +599,8 @@ int drm_pagemap_migrate_to_devmem(struct drm_pagemap_devmem *devmem_allocation,
goto err_free;
}

- if (migrate.cpages != npages) {
+ if (migrate.cpages != npages &&
+ drm_pagemap_cpages(migrate.src, npages) != npages) {
/*
* Some pages to migrate. But we want to migrate all or
* nothing. Raced or unknown device pages.
--
2.43.0