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

On 20-08-2026 12:33, Matthew Brost wrote:
> On Sun, Aug 16, 2026 at 08:44:21PM +0530, Ghimiray, Himal Prasad wrote:
>>
>>
>> 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.
> 
> I checked on this and kernel wide no code seems to undo
> fault_create_debugfs_attr on module unload, nor is there a function in
> linux/fault-inject.h to undo all debugfs entries setup.

I believe the cleanup is done via standard debugfs_remove_recursive via 
passing the fault_create_debugfs_attr dir or parent. Here we have no 
parent so

dir = fault_create_debugfs_attr at init

and debugfs_remove_recursive(dir) should be sufficient during 
drm_pagemap_exit

I assume not cleaning it exit might leave the debugfs entries incase of 
module unload.
  >
> So IMO this is either everyone is kernel is doing this wrong or this is
> a non-issue. debugfs_create_file kernel doc seems to indicate all
> debugfs enteries should be removed with debugfs_remove though (?).
> Either way I'd say this out of scope for this series.
> 
> Matt
> 
>>>    	flush_work(&drm_pagemap_work);
>>