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Wed, 21 Jan 2026 04:04:40 +0000 Received: from DM4PR12MB9072.namprd12.prod.outlook.com ([fe80::9e49:782:8e98:1ff1]) by DM4PR12MB9072.namprd12.prod.outlook.com ([fe80::9e49:782:8e98:1ff1%5]) with mapi id 15.20.9520.011; Wed, 21 Jan 2026 04:04:39 +0000 Message-ID: <254bd66c-4c0f-44f4-a4a1-87dc44bc5e30@nvidia.com> Date: Wed, 21 Jan 2026 15:04:29 +1100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 11/11] mm: Remove device private pages from the physical address space To: Zi Yan Cc: linux-mm@kvack.org, balbirs@nvidia.com, matthew.brost@intel.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, david@redhat.com, apopple@nvidia.com, lorenzo.stoakes@oracle.com, lyude@redhat.com, dakr@kernel.org, airlied@gmail.com, simona@ffwll.ch, rcampbell@nvidia.com, mpenttil@redhat.com, jgg@nvidia.com, willy@infradead.org, linuxppc-dev@lists.ozlabs.org, intel-xe@lists.freedesktop.org, jgg@ziepe.ca, Felix.Kuehling@amd.com References: <20260107091823.68974-1-jniethe@nvidia.com> <20260107091823.68974-12-jniethe@nvidia.com> <36F96303-8FBB-4350-9472-52CC50BAB956@nvidia.com> <6C5F185E-BB12-4B01-8283-F2C956E84AA3@nvidia.com> <16770FCE-A248-4184-ABFC-94C02C0B30F3@nvidia.com> <649cc20c-161c-4343-8263-3712a4f6dccb@nvidia.com> Content-Language: en-US From: Jordan Niethe In-Reply-To: Content-Type: text/plain; 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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 >>>>>>>> Signed-off-by: Alistair Popple >>>>>>>> >>>>>>>> --- >>>>>>>> >>>>>>>> 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(-) >>>>>>>> >>>>>>> >>>>>>> >>>>>>>> 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); >>>>>>>> } >>>>>>> >>>>>>> >>>>>>> >>>>>>>> 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