Re: [PATCH v2 11/11] mm: Remove device private pages from the physical address space

[Jordan Niethe <jniethe@nvidia.com>]

Hi,

On 21/1/26 09:53, Zi Yan wrote:
> On 20 Jan 2026, at 17:33, Jordan Niethe wrote:
> 
>> On 14/1/26 07:04, Zi Yan wrote:
>>> On 7 Jan 2026, at 4:18, Jordan Niethe wrote:
>>>
>>>> Currently when creating these device private struct pages, the first
>>>> step is to use request_free_mem_region() to get a range of physical
>>>> address space large enough to represent the devices memory. This
>>>> allocated physical address range is then remapped as device private
>>>> memory using memremap_pages().
>>>>
>>>> Needing allocation of physical address space has some problems:
>>>>
>>>>     1) There may be insufficient physical address space to represent the
>>>>        device memory. KASLR reducing the physical address space and VM
>>>>        configurations with limited physical address space increase the
>>>>        likelihood of hitting this especially as device memory increases. This
>>>>        has been observed to prevent device private from being initialized.
>>>>
>>>>     2) Attempting to add the device private pages to the linear map at
>>>>        addresses beyond the actual physical memory causes issues on
>>>>        architectures like aarch64 meaning the feature does not work there.
>>>>
>>>> Instead of using the physical address space, introduce a device private
>>>> address space and allocate devices regions from there to represent the
>>>> device private pages.
>>>>
>>>> Introduce a new interface memremap_device_private_pagemap() that
>>>> allocates a requested amount of device private address space and creates
>>>> the necessary device private pages.
>>>>
>>>> To support this new interface, struct dev_pagemap needs some changes:
>>>>
>>>>     - Add a new dev_pagemap::nr_pages field as an input parameter.
>>>>     - Add a new dev_pagemap::pages array to store the device
>>>>       private pages.
>>>>
>>>> When using memremap_device_private_pagemap(), rather then passing in
>>>> dev_pagemap::ranges[dev_pagemap::nr_ranges] of physical address space to
>>>> be remapped, dev_pagemap::nr_ranges will always be 1, and the device
>>>> private range that is reserved is returned in dev_pagemap::range.
>>>>
>>>> Forbid calling memremap_pages() with dev_pagemap::ranges::type =
>>>> MEMORY_DEVICE_PRIVATE.
>>>>
>>>> Represent this device private address space using a new
>>>> device_private_pgmap_tree maple tree. This tree maps a given device
>>>> private address to a struct dev_pagemap, where a specific device private
>>>> page may then be looked up in that dev_pagemap::pages array.
>>>>
>>>> Device private address space can be reclaimed and the assoicated device
>>>> private pages freed using the corresponding new
>>>> memunmap_device_private_pagemap() interface.
>>>>
>>>> Because the device private pages now live outside the physical address
>>>> space, they no longer have a normal PFN. This means that page_to_pfn(),
>>>> et al. are no longer meaningful.
>>>>
>>>> Introduce helpers:
>>>>
>>>>     - device_private_page_to_offset()
>>>>     - device_private_folio_to_offset()
>>>>
>>>> to take a given device private page / folio and return its offset within
>>>> the device private address space.
>>>>
>>>> Update the places where we previously converted a device private page to
>>>> a PFN to use these new helpers. When we encounter a device private
>>>> offset, instead of looking up its page within the pagemap use
>>>> device_private_offset_to_page() instead.
>>>>
>>>> Update the existing users:
>>>>
>>>>    - lib/test_hmm.c
>>>>    - ppc ultravisor
>>>>    - drm/amd/amdkfd
>>>>    - gpu/drm/xe
>>>>    - gpu/drm/nouveau
>>>>
>>>> to use the new memremap_device_private_pagemap() interface.
>>>>
>>>> Signed-off-by: Jordan Niethe <jniethe@nvidia.com>
>>>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>>>>
>>>> ---
>>>>
>>>> NOTE: The updates to the existing drivers have only been compile tested.
>>>> I'll need some help in testing these drivers.
>>>>
>>>> v1:
>>>> - Include NUMA node paramater for memremap_device_private_pagemap()
>>>> - Add devm_memremap_device_private_pagemap() and friends
>>>> - Update existing users of memremap_pages():
>>>>       - ppc ultravisor
>>>>       - drm/amd/amdkfd
>>>>       - gpu/drm/xe
>>>>       - gpu/drm/nouveau
>>>> - Update for HMM huge page support
>>>> - Guard device_private_offset_to_page and friends with CONFIG_ZONE_DEVICE
>>>>
>>>> v2:
>>>> - Make sure last member of struct dev_pagemap remains DECLARE_FLEX_ARRAY(struct range, ranges);
>>>> ---
>>>>    Documentation/mm/hmm.rst                 |  11 +-
>>>>    arch/powerpc/kvm/book3s_hv_uvmem.c       |  41 ++---
>>>>    drivers/gpu/drm/amd/amdkfd/kfd_migrate.c |  23 +--
>>>>    drivers/gpu/drm/nouveau/nouveau_dmem.c   |  35 ++--
>>>>    drivers/gpu/drm/xe/xe_svm.c              |  28 +---
>>>>    include/linux/hmm.h                      |   3 +
>>>>    include/linux/leafops.h                  |  16 +-
>>>>    include/linux/memremap.h                 |  64 +++++++-
>>>>    include/linux/migrate.h                  |   6 +-
>>>>    include/linux/mm.h                       |   2 +
>>>>    include/linux/rmap.h                     |   5 +-
>>>>    include/linux/swapops.h                  |  10 +-
>>>>    lib/test_hmm.c                           |  69 ++++----
>>>>    mm/debug.c                               |   9 +-
>>>>    mm/memremap.c                            | 193 ++++++++++++++++++-----
>>>>    mm/mm_init.c                             |   8 +-
>>>>    mm/page_vma_mapped.c                     |  19 ++-
>>>>    mm/rmap.c                                |  43 +++--
>>>>    mm/util.c                                |   5 +-
>>>>    19 files changed, 391 insertions(+), 199 deletions(-)
>>>>
>>> <snip>
>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index e65329e1969f..b36599ab41ba 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -2038,6 +2038,8 @@ static inline unsigned long memdesc_section(memdesc_flags_t mdf)
>>>>     */
>>>>    static inline unsigned long folio_pfn(const struct folio *folio)
>>>>    {
>>>> +	VM_BUG_ON(folio_is_device_private(folio));
>>>
>>> Please use VM_WARN_ON instead.
>>
>> ack.
>>
>>>
>>>> +
>>>>    	return page_to_pfn(&folio->page);
>>>>    }
>>>>
>>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>>> index 57c63b6a8f65..c1561a92864f 100644
>>>> --- a/include/linux/rmap.h
>>>> +++ b/include/linux/rmap.h
>>>> @@ -951,7 +951,7 @@ static inline unsigned long page_vma_walk_pfn(unsigned long pfn)
>>>>    static inline unsigned long folio_page_vma_walk_pfn(const struct folio *folio)
>>>>    {
>>>>    	if (folio_is_device_private(folio))
>>>> -		return page_vma_walk_pfn(folio_pfn(folio)) |
>>>> +		return page_vma_walk_pfn(device_private_folio_to_offset(folio)) |
>>>>    		       PVMW_PFN_DEVICE_PRIVATE;
>>>>
>>>>    	return page_vma_walk_pfn(folio_pfn(folio));
>>>> @@ -959,6 +959,9 @@ static inline unsigned long folio_page_vma_walk_pfn(const struct folio *folio)
>>>>
>>>>    static inline struct page *page_vma_walk_pfn_to_page(unsigned long pvmw_pfn)
>>>>    {
>>>> +	if (pvmw_pfn & PVMW_PFN_DEVICE_PRIVATE)
>>>> +		return device_private_offset_to_page(pvmw_pfn >> PVMW_PFN_SHIFT);
>>>> +
>>>>    	return pfn_to_page(pvmw_pfn >> PVMW_PFN_SHIFT);
>>>>    }
>>>
>>> <snip>
>>>
>>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>>> index 96c525785d78..141fe5abd33f 100644
>>>> --- a/mm/page_vma_mapped.c
>>>> +++ b/mm/page_vma_mapped.c
>>>> @@ -107,6 +107,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>>>    static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>>>    {
>>>>    	unsigned long pfn;
>>>> +	bool device_private = false;
>>>>    	pte_t ptent = ptep_get(pvmw->pte);
>>>>
>>>>    	if (pvmw->flags & PVMW_MIGRATION) {
>>>> @@ -115,6 +116,9 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>>>    		if (!softleaf_is_migration(entry))
>>>>    			return false;
>>>>
>>>> +		if (softleaf_is_migration_device_private(entry))
>>>> +			device_private = true;
>>>> +
>>>>    		pfn = softleaf_to_pfn(entry);
>>>>    	} else if (pte_present(ptent)) {
>>>>    		pfn = pte_pfn(ptent);
>>>> @@ -127,8 +131,14 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>>>    			return false;
>>>>
>>>>    		pfn = softleaf_to_pfn(entry);
>>>> +
>>>> +		if (softleaf_is_device_private(entry))
>>>> +			device_private = true;
>>>>    	}
>>>>
>>>> +	if ((device_private) ^ !!(pvmw->pfn & PVMW_PFN_DEVICE_PRIVATE))
>>>> +		return false;
>>>> +
>>>>    	if ((pfn + pte_nr - 1) < (pvmw->pfn >> PVMW_PFN_SHIFT))
>>>>    		return false;
>>>>    	if (pfn > ((pvmw->pfn >> PVMW_PFN_SHIFT) + pvmw->nr_pages - 1))
>>>> @@ -137,8 +147,11 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>>>    }
>>>>
>>>>    /* Returns true if the two ranges overlap.  Careful to not overflow. */
>>>> -static bool check_pmd(unsigned long pfn, struct page_vma_mapped_walk *pvmw)
>>>> +static bool check_pmd(unsigned long pfn, bool device_private, struct page_vma_mapped_walk *pvmw)
>>>>    {
>>>> +	if ((device_private) ^ !!(pvmw->pfn & PVMW_PFN_DEVICE_PRIVATE))
>>>> +		return false;
>>>> +
>>>>    	if ((pfn + HPAGE_PMD_NR - 1) < (pvmw->pfn >> PVMW_PFN_SHIFT))
>>>>    		return false;
>>>>    	if (pfn > (pvmw->pfn >> PVMW_PFN_SHIFT) + pvmw->nr_pages - 1)
>>>> @@ -255,6 +268,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>>
>>>>    				if (!softleaf_is_migration(entry) ||
>>>>    				    !check_pmd(softleaf_to_pfn(entry),
>>>> +					       softleaf_is_device_private(entry) ||
>>>> +					       softleaf_is_migration_device_private(entry),
>>>>    					       pvmw))
>>>>    					return not_found(pvmw);
>>>>    				return true;
>>>> @@ -262,7 +277,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>>    			if (likely(pmd_trans_huge(pmde))) {
>>>>    				if (pvmw->flags & PVMW_MIGRATION)
>>>>    					return not_found(pvmw);
>>>> -				if (!check_pmd(pmd_pfn(pmde), pvmw))
>>>> +				if (!check_pmd(pmd_pfn(pmde), false, pvmw))
>>>>    					return not_found(pvmw);
>>>>    				return true;
>>>>    			}
>>>
>>> It seems to me that you can add a new flag like “bool is_device_private” to
>>> indicate whether pfn is a device private index instead of pfn without
>>> manipulating pvmw->pfn itself.
>>
>> We could do it like that, however my concern with using a new param was that
>> storing this info seperately might make it easier to misuse a device private
>> index as a regular pfn.
>>
>> It seemed like it could be easy to overlook both when creating the pvmw and
>> then when accessing the pfn.
> 
> That is why I asked for a helper function like page_vma_walk_pfn(pvmw) to
> return the converted pfn instead of pvmw->pfn directly. You can add a comment
> to ask people to use helper function and even mark pvmw->pfn /* do not use
> directly */.

Yeah I agree that is a good idea.

> 
> In addition, your patch manipulates pfn by left shifting it by 1. Are you sure
> there is no weird arch having pfns with bit 63 being 1? Your change could
> break it, right?

Currently for migrate pfns we left shift by pfns by MIGRATE_PFN_SHIFT (6), so I
thought doing something similiar here should be safe.

Thanks,
Jordan.

> 
> 
> Best Regards,
> Yan, Zi