Re: [PATCH] drm/pagemap: Guard HPAGE_PMD_ORDER use with CONFIG_ARCH_ENABLE_THP_MIGRATION

[Matthew Brost]

On Tue, Jul 21, 2026 at 12:40:38PM +0200, Jan Stancek wrote:
> HPAGE_PMD_SHIFT expands to BUILD_BUG() when CONFIG_PGTABLE_HAS_HUGE_LEAVES
> is not set, causing a compile error when both CONFIG_TRANSPARENT_HUGEPAGE
> and CONFIG_HUGETLB_PAGE are disabled:
> 
>  drivers/gpu/drm/drm_pagemap.c:480:12: error: call to '__compiletime_assert_458'
>  declared with 'error' attribute: BUILD_BUG failed
>    480 |                         order = HPAGE_PMD_ORDER;
>        |                                 ^
>  include/linux/huge_mm.h:117:26: note: expanded from macro 'HPAGE_PMD_ORDER'
>    117 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
>        |                          ^
>  include/linux/huge_mm.h:113:28: note: expanded from macro 'HPAGE_PMD_SHIFT'
>    113 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> 
> Guard the two HPAGE_PMD_ORDER uses with CONFIG_ARCH_ENABLE_THP_MIGRATION,
> matching the pattern used in migrate_device.c.
> 
> Fixes: 139ab31aea8a ("drm/pagemap: Correct cpages calculation for migrate_vma_setup")
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/r/202607201914.LpAGsbXs-lkp@xxxxxxxxx/
> Signed-off-by: Jan Stancek <jstancek@xxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_pagemap.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> index 15c78eca180b..53a78752c8be 100644
> --- a/drivers/gpu/drm/drm_pagemap.c
> +++ b/drivers/gpu/drm/drm_pagemap.c
> @@ -578,10 +578,13 @@ static int drm_pagemap_cpages(unsigned long *migrate_pfn, unsigned long npages)
>  			folio = page_folio(page);
>  			order = folio_order(folio);
>  			cpages += NR_PAGES(order);
> -		} else if (migrate_pfn[i] & MIGRATE_PFN_COMPOUND) {
> +		}
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +		else if (migrate_pfn[i] & MIGRATE_PFN_COMPOUND) {
>  			order = HPAGE_PMD_ORDER;

Would it be do something like this at the top of the file...

#ifdef HPAGE_PMD_ORDER
#define DRM_PAGEMAP_PMD_ORDER	HPAGE_PMD_ORDER
#else
#define DRM_PAGEMAP_PMD_ORDER	(-1)
#endif

Then in common code s/HPAGE_PMD_ORDER/DRM_PAGEMAP_PMD_ORDER/

I think this works as we can never get to the MIGRATE_PFN_COMPOUND code
paths without CONFIG_ARCH_ENABLE_THP_MIGRATION enabled (see
mm/migrate_device.c).

Also btw fixing the unrelated bug Sashiko flagged in this patch.

Matt

>  			cpages += NR_PAGES(order);
>  		}
> +#endif
>  
>  		i += NR_PAGES(order);
>  	}
> @@ -733,7 +736,6 @@ int drm_pagemap_migrate_to_devmem(struct drm_pagemap_devmem *devmem_allocation,
>  	own_pages = 0;
>  
>  	for (i = 0; i < npages;) {
> -		unsigned long j;
>  		struct page *page = pfn_to_page(migrate.dst[i]);
>  		struct page *src_page = migrate_pfn_to_page(migrate.src[i]);
>  		unsigned int order = 0;
> @@ -763,7 +765,10 @@ int drm_pagemap_migrate_to_devmem(struct drm_pagemap_devmem *devmem_allocation,
>  		}
>  		migrate.dst[i] = migrate_pfn(migrate.dst[i]);
>  
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
>  		if (migrate.src[i] & MIGRATE_PFN_COMPOUND) {
> +			unsigned long j;
> +
>  			drm_WARN_ONCE(dpagemap->drm, src_page &&
>  				      folio_order(page_folio(src_page)) != HPAGE_PMD_ORDER,
>  				      "Unexpected folio order\n");
> @@ -774,6 +779,7 @@ int drm_pagemap_migrate_to_devmem(struct drm_pagemap_devmem *devmem_allocation,
>  			for (j = 1; j < NR_PAGES(order) && i + j < npages; j++)
>  				migrate.dst[i + j] = 0;
>  		}
> +#endif
>  
>  		drm_pagemap_get_devmem_page(page, order, zdd);
>  
> -- 
> 2.52.0
>