Re: [PATCH v4 6/7] drm/pagemap: Correct cpages calculation for migrate_vma_setup

[Francois Dugast <francois.dugast@intel.com>]

On Sun, Jan 11, 2026 at 09:55:45PM +0100, Francois Dugast wrote:
> 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>
> Signed-off-by: Francois Dugast <francois.dugast@intel.com>

Reviewed-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 308c14291eba..af2c8f4da00e 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
>