Re: [PATCH v3 6/6] drm/pagemap: Add fault injection for higher-order RAM folio allocation

On 06-08-2026 04:40, Matthew Brost wrote:
> Migrating a device-private THP back to system memory has two distinct
> paths in __migrate_device_pages(): the fast path where both source and
> destination carry MIGRATE_PFN_COMPOUND, and the fallback path where the
> destination could only be satisfied with order-0 folios and the source
> THP therefore has to be split via migrate_vma_split_unmapped_folio().
> 
> The fallback path only triggers under genuine memory pressure, which
> makes it both rare and awkward to reproduce, yet it is the path where
> the interesting refcounting happens (the CPU fault holds an extra
> reference on the device folio taken by do_huge_pmd_device_private()).
> 
> Add a fault_attr, modelled on backup_fault_inject in ttm_pool.c, that
> forces the higher-order allocation in
> drm_pagemap_migrate_populate_ram_pfn() to fail so the existing order-0
> fallback is taken deterministically.
> 
> The attribute is exposed at /sys/kernel/debug/drm_pagemap_fault_inject
> and requires CONFIG_FAULT_INJECTION_DEBUG_FS. With
> CONFIG_FAULT_INJECTION disabled the helper compiles out to a constant
> false and the injection has no cost.
> 
> Cc: Andrew Morton <[email protected]>
> Cc: David Hildenbrand <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: Zi Yan <[email protected]>
> Cc: Baolin Wang <[email protected]>
> Cc: Liam R. Howlett <[email protected]>
> Cc: Nico Pache <[email protected]>
> Cc: Ryan Roberts <[email protected]>
> Cc: Dev Jain <[email protected]>
> Cc: Barry Song <[email protected]>
> Cc: Lance Yang <[email protected]>
> Cc: Usama Arif <[email protected]>
> Cc: Joshua Hahn <[email protected]>
> Cc: Rakie Kim <[email protected]>
> Cc: Byungchul Park <[email protected]>
> Cc: Gregory Price <[email protected]>
> Cc: Ying Huang <[email protected]>
> Cc: Alistair Popple <[email protected]>
> Cc: Balbir Singh <[email protected]>
> Cc: Maarten Lankhorst <[email protected]>
> Cc: Maxime Ripard <[email protected]>
> Cc: Thomas Zimmermann <[email protected]>
> Cc: David Airlie <[email protected]>
> Cc: Simona Vetter <[email protected]>
> Cc: Thomas Hellström <[email protected]>
> Cc: Francois Dugast <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Assisted-by: GitHub_Copilot:claude-opus-5
> Signed-off-by: Matthew Brost <[email protected]>
> ---
>   drivers/gpu/drm/drm_pagemap.c | 36 ++++++++++++++++++++++++++++++++++-
>   1 file changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> index 51c6f12e4256..6ae8c9aa36cc 100644
> --- a/drivers/gpu/drm/drm_pagemap.c
> +++ b/drivers/gpu/drm/drm_pagemap.c
> @@ -3,6 +3,7 @@
>    * Copyright © 2024-2025 Intel Corporation
>    */
>   
> +#include <linux/debugfs.h>
>   #include <linux/dma-fence.h>
>   #include <linux/dma-mapping.h>
>   #include <linux/migrate.h>
> @@ -12,6 +13,27 @@
>   #include <drm/drm_pagemap_util.h>
>   #include <drm/drm_print.h>
>   
> +#ifdef CONFIG_FAULT_INJECTION
> +#include <linux/fault-inject.h>
> +static DECLARE_FAULT_ATTR(migrate_to_ram_fault_inject);
> +
> +/*
> + * Force a higher-order destination folio allocation to fail in
> + * drm_pagemap_migrate_populate_ram_pfn(), exercising the order-0 fallback
> + * (and, in turn, the THP split path in __migrate_device_pages()) without
> + * having to drive the system into actual memory pressure.
> + */
> +static bool drm_pagemap_fault_inject_folio(void)
> +{
> +	return should_fail(&migrate_to_ram_fault_inject, 1);
> +}
> +#else
> +static bool drm_pagemap_fault_inject_folio(void)
> +{
> +	return false;
> +}
> +#endif
> +
>   /**
>    * DOC: Overview
>    *
> @@ -960,7 +982,9 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
>   		if (order)
>   			gfp |= __GFP_NOWARN;
>   
> -		if (vas)
> +		if (order && drm_pagemap_fault_inject_folio())
> +			folio = NULL;
> +		else if (vas)
>   			folio = vma_alloc_folio(gfp, order, vas, addr);
>   		else
>   			folio = folio_alloc(gfp, order);
> @@ -1554,6 +1578,16 @@ void drm_pagemap_destroy(struct drm_pagemap *dpagemap, bool is_atomic_or_reclaim
>   		kfree(dpagemap);
>   }
>   
> +static int __init drm_pagemap_module_init(void)
> +{
> +#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_FAULT_INJECTION)
> +	fault_create_debugfs_attr("drm_pagemap_fault_inject", NULL,
> +				  &migrate_to_ram_fault_inject);
> +#endif
> +	return 0;
> +}
> +module_init(drm_pagemap_module_init);
> +
>   static void drm_pagemap_exit(void)
>   {

Missed fault injection debugfs removal ?

Sashiko flags it and looks valid concern.
>   	flush_work(&drm_pagemap_work);