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Message-ID: <254bd66c-4c0f-44f4-a4a1-87dc44bc5e30@nvidia.com>
Date: Wed, 21 Jan 2026 15:04:29 +1100
From: Jordan Niethe <jniethe@...dia.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, balbirs@...dia.com, matthew.brost@...el.com,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
dri-devel@...ts.freedesktop.org, david@...hat.com, apopple@...dia.com,
lorenzo.stoakes@...cle.com, lyude@...hat.com, dakr@...nel.org,
airlied@...il.com, simona@...ll.ch, rcampbell@...dia.com,
mpenttil@...hat.com, jgg@...dia.com, willy@...radead.org,
linuxppc-dev@...ts.ozlabs.org, intel-xe@...ts.freedesktop.org, jgg@...pe.ca,
Felix.Kuehling@....com
Subject: Re: [PATCH v2 11/11] mm: Remove device private pages from the
physical address space
On 21/1/26 13:41, Zi Yan wrote:
> On 20 Jan 2026, at 18:34, Jordan Niethe wrote:
>
>> Hi,
>>
>> On 21/1/26 10:06, Zi Yan wrote:
>>> On 20 Jan 2026, at 18:02, Jordan Niethe wrote:
>>>
>>>> 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@...dia.com>
>>>>>>>> Signed-off-by: Alistair Popple <apopple@...dia.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.
>>>
>>> Yeah, but that limits to archs supporting HMM. page_vma_mapped_walk is used
>>> by almost every arch, so it has a broader impact.
>>
>> That is a good point.
>>
>> I see a few options:
>>
>> - On every arch we can assume SWP_PFN_BITS? I could add a sanity check that we
>> have an extra bit on top of SWP_PFN_BITS within an unsigned long.
>
> Yes, but if there is no extra bit, are you going to disable device private
> pages?
In this case, migrate PFNs would also be broken (due to MIGRATE_PFN_SHIFT) so we'd have to.
>
>> - We could define PVMW_PFN_SHIFT as 0 if !CONFIG_MIGRATION as the flag is not
>> required.
>
> Sure, or !CONFIG_DEVICE_MIGRATION
>
>> - Instead of modifying pvmw->pfn we could use pvmw->flags but that has the
>> issues of separating the offset type and offset.
>
> It seems that I was not clear on my proposal. Here is the patch on top of
> your patchset and it compiles.
Oh I'd interpreted “bool is_device_private” as adding a new field to pvmw.
>
> Basically, pvmw->pfn stores either PFN or device private offset without
> additional shift. Caller interprets pvmw->pfn based on
> pvmw->flags & PVMW_DEVICE_PRIVATE. And you can ignore my helper function
> of pvmw->pfn suggestion, since my patch below can use pvmw->pfn directly.
Thanks, looks reasonable. I'll try it.
Thanks,
Jordan.
>
> Let me know if my patch works. Thanks.
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index c1561a92864f..4423f0e886aa 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -921,6 +921,7 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
> #define PVMW_SYNC (1 << 0)
> /* Look for migration entries rather than present PTEs */
> #define PVMW_MIGRATION (1 << 1)
> +#define PVMW_DEVICE_PRIVATE (1 << 2)
>
> /* Result flags */
>
> @@ -943,6 +944,13 @@ struct page_vma_mapped_walk {
> #define PVMW_PFN_DEVICE_PRIVATE (1UL << 0)
> #define PVMW_PFN_SHIFT 1
>
> +static inline unsigned long page_vma_walk_flags(struct folio *folio, unsigned long flags)
> +{
> + if (folio_is_device_private(folio))
> + return flags | PVMW_DEVICE_PRIVATE;
> + return flags;
> +}
> +
> static inline unsigned long page_vma_walk_pfn(unsigned long pfn)
> {
> return (pfn << PVMW_PFN_SHIFT);
> @@ -951,23 +959,16 @@ 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(device_private_folio_to_offset(folio)) |
> - PVMW_PFN_DEVICE_PRIVATE;
> -
> - return page_vma_walk_pfn(folio_pfn(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 device_private_folio_to_offset(folio);
>
> - return pfn_to_page(pvmw_pfn >> PVMW_PFN_SHIFT);
> + return (folio_pfn(folio));
> }
>
> -static inline struct folio *page_vma_walk_pfn_to_folio(unsigned long pvmw_pfn)
> +static inline struct folio *page_vma_walk_pfn_to_folio(struct page_vma_mapped_walk *pvmw)
> {
> - return page_folio(page_vma_walk_pfn_to_page(pvmw_pfn));
> + if (pvmw->flags & PVMW_DEVICE_PRIVATE)
> + return page_folio(device_private_offset_to_page(pvmw->pfn));
> + return pfn_folio(pvmw->pfn);
> }
>
> #define DEFINE_FOLIO_VMA_WALK(name, _folio, _vma, _address, _flags) \
> @@ -977,7 +978,7 @@ static inline struct folio *page_vma_walk_pfn_to_folio(unsigned long pvmw_pfn)
> .pgoff = folio_pgoff(_folio), \
> .vma = _vma, \
> .address = _address, \
> - .flags = _flags, \
> + .flags = page_vma_walk_flags(_folio, _flags), \
> }
>
> static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 141fe5abd33f..e61a0e49a7c9 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -136,12 +136,12 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
> device_private = true;
> }
>
> - if ((device_private) ^ !!(pvmw->pfn & PVMW_PFN_DEVICE_PRIVATE))
> + if ((device_private) ^ !!(pvmw->flags & PVMW_DEVICE_PRIVATE))
> return false;
>
> - if ((pfn + pte_nr - 1) < (pvmw->pfn >> PVMW_PFN_SHIFT))
> + if ((pfn + pte_nr - 1) < pvmw->pfn)
> return false;
> - if (pfn > ((pvmw->pfn >> PVMW_PFN_SHIFT) + pvmw->nr_pages - 1))
> + if (pfn > (pvmw->pfn + pvmw->nr_pages - 1))
> return false;
> return true;
> }
> @@ -149,12 +149,12 @@ 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, bool device_private, struct page_vma_mapped_walk *pvmw)
> {
> - if ((device_private) ^ !!(pvmw->pfn & PVMW_PFN_DEVICE_PRIVATE))
> + if ((device_private) ^ !!(pvmw->flags & PVMW_DEVICE_PRIVATE))
> return false;
>
> - if ((pfn + HPAGE_PMD_NR - 1) < (pvmw->pfn >> PVMW_PFN_SHIFT))
> + if ((pfn + HPAGE_PMD_NR - 1) < pvmw->pfn)
> return false;
> - if (pfn > (pvmw->pfn >> PVMW_PFN_SHIFT) + pvmw->nr_pages - 1)
> + if (pfn > pvmw->pfn + pvmw->nr_pages - 1)
> return false;
> return true;
> }
> @@ -369,7 +369,7 @@ unsigned long page_mapped_in_vma(const struct page *page,
> .pfn = folio_page_vma_walk_pfn(folio),
> .nr_pages = 1,
> .vma = vma,
> - .flags = PVMW_SYNC,
> + .flags = page_vma_walk_flags(folio, PVMW_SYNC),
> };
>
> pvmw.address = vma_address(vma, page_pgoff(folio, page), 1);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index be5682d345b5..5d81939bf12a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4203,7 +4203,7 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
> pte_t *pte = pvmw->pte;
> unsigned long addr = pvmw->address;
> struct vm_area_struct *vma = pvmw->vma;
> - struct folio *folio = page_vma_walk_pfn_to_folio(pvmw->pfn);
> + struct folio *folio = page_vma_walk_pfn_to_folio(pvmw);
> struct mem_cgroup *memcg = folio_memcg(folio);
> struct pglist_data *pgdat = folio_pgdat(folio);
> struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
>
>
>
>
>
> Best Regards,
> Yan, Zi
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