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[2003:d8:2f44:3700:be07:9a67:67f7:24e6]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c3abec8sm3424614f8f.8.2025.07.31.12.09.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 31 Jul 2025 12:09:43 -0700 (PDT) Message-ID: <6c85ebe8-55f9-4f1e-8da0-5a3587a047d4@redhat.com> Date: Thu, 31 Jul 2025 21:09:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code To: Zi Yan Cc: =?UTF-8?Q?Mika_Penttil=C3=A4?= , Balbir Singh , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Shuah Khan , Barry Song , Baolin Wang , Ryan Roberts , Matthew Wilcox , Peter Xu , Kefeng Wang , Jane Chu , Alistair Popple , Donet Tom , Matthew Brost , Francois Dugast , Ralph Campbell References: <20250730092139.3890844-1-balbirs@nvidia.com> <20250730092139.3890844-3-balbirs@nvidia.com> <22D1AD52-F7DA-4184-85A7-0F14D2413591@nvidia.com> <9f836828-4f53-41a0-b5f7-bbcd2084086e@redhat.com> <884b9246-de7c-4536-821f-1bf35efe31c8@redhat.com> <6291D401-1A45-4203-B552-79FE26E151E4@nvidia.com> <8E2CE1DF-4C37-4690-B968-AEA180FF44A1@nvidia.com> <2308291f-3afc-44b4-bfc9-c6cf0cdd6295@redhat.com> <9FBDBFB9-8B27-459C-8047-055F90607D60@nvidia.com> <11ee9c5e-3e74-4858-bf8d-94daf1530314@redhat.com> <182044F2-657E-4FFA-AED8-225304AAD2FB@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 31.07.25 15:34, Zi Yan wrote: > On 31 Jul 2025, at 8:32, David Hildenbrand wrote: > >> On 31.07.25 13:26, Zi Yan wrote: >>> On 31 Jul 2025, at 3:15, David Hildenbrand wrote: >>> >>>> On 30.07.25 18:29, Mika Penttilä wrote: >>>>> >>>>> On 7/30/25 18:58, Zi Yan wrote: >>>>>> On 30 Jul 2025, at 11:40, Mika Penttilä wrote: >>>>>> >>>>>>> On 7/30/25 18:10, Zi Yan wrote: >>>>>>>> On 30 Jul 2025, at 8:49, Mika Penttilä wrote: >>>>>>>> >>>>>>>>> On 7/30/25 15:25, Zi Yan wrote: >>>>>>>>>> On 30 Jul 2025, at 8:08, Mika Penttilä wrote: >>>>>>>>>> >>>>>>>>>>> On 7/30/25 14:42, Mika Penttilä wrote: >>>>>>>>>>>> On 7/30/25 14:30, Zi Yan wrote: >>>>>>>>>>>>> On 30 Jul 2025, at 7:27, Zi Yan wrote: >>>>>>>>>>>>> >>>>>>>>>>>>>> On 30 Jul 2025, at 7:16, Mika Penttilä wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>>> Hi, >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On 7/30/25 12:21, Balbir Singh wrote: >>>>>>>>>>>>>>>> Make THP handling code in the mm subsystem for THP pages aware of zone >>>>>>>>>>>>>>>> device pages. Although the code is designed to be generic when it comes >>>>>>>>>>>>>>>> to handling splitting of pages, the code is designed to work for THP >>>>>>>>>>>>>>>> page sizes corresponding to HPAGE_PMD_NR. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Modify page_vma_mapped_walk() to return true when a zone device huge >>>>>>>>>>>>>>>> entry is present, enabling try_to_migrate() and other code migration >>>>>>>>>>>>>>>> paths to appropriately process the entry. page_vma_mapped_walk() will >>>>>>>>>>>>>>>> return true for zone device private large folios only when >>>>>>>>>>>>>>>> PVMW_THP_DEVICE_PRIVATE is passed. This is to prevent locations that are >>>>>>>>>>>>>>>> not zone device private pages from having to add awareness. The key >>>>>>>>>>>>>>>> callback that needs this flag is try_to_migrate_one(). The other >>>>>>>>>>>>>>>> callbacks page idle, damon use it for setting young/dirty bits, which is >>>>>>>>>>>>>>>> not significant when it comes to pmd level bit harvesting. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> pmd_pfn() does not work well with zone device entries, use >>>>>>>>>>>>>>>> pfn_pmd_entry_to_swap() for checking and comparison as for zone device >>>>>>>>>>>>>>>> entries. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Zone device private entries when split via munmap go through pmd split, >>>>>>>>>>>>>>>> but need to go through a folio split, deferred split does not work if a >>>>>>>>>>>>>>>> fault is encountered because fault handling involves migration entries >>>>>>>>>>>>>>>> (via folio_migrate_mapping) and the folio sizes are expected to be the >>>>>>>>>>>>>>>> same there. This introduces the need to split the folio while handling >>>>>>>>>>>>>>>> the pmd split. Because the folio is still mapped, but calling >>>>>>>>>>>>>>>> folio_split() will cause lock recursion, the __split_unmapped_folio() >>>>>>>>>>>>>>>> code is used with a new helper to wrap the code >>>>>>>>>>>>>>>> split_device_private_folio(), which skips the checks around >>>>>>>>>>>>>>>> folio->mapping, swapcache and the need to go through unmap and remap >>>>>>>>>>>>>>>> folio. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Cc: Karol Herbst >>>>>>>>>>>>>>>> Cc: Lyude Paul >>>>>>>>>>>>>>>> Cc: Danilo Krummrich >>>>>>>>>>>>>>>> Cc: David Airlie >>>>>>>>>>>>>>>> Cc: Simona Vetter >>>>>>>>>>>>>>>> Cc: "Jérôme Glisse" >>>>>>>>>>>>>>>> Cc: Shuah Khan >>>>>>>>>>>>>>>> Cc: David Hildenbrand >>>>>>>>>>>>>>>> Cc: Barry Song >>>>>>>>>>>>>>>> Cc: Baolin Wang >>>>>>>>>>>>>>>> Cc: Ryan Roberts >>>>>>>>>>>>>>>> Cc: Matthew Wilcox >>>>>>>>>>>>>>>> Cc: Peter Xu >>>>>>>>>>>>>>>> Cc: Zi Yan >>>>>>>>>>>>>>>> Cc: Kefeng Wang >>>>>>>>>>>>>>>> Cc: Jane Chu >>>>>>>>>>>>>>>> Cc: Alistair Popple >>>>>>>>>>>>>>>> Cc: Donet Tom >>>>>>>>>>>>>>>> Cc: Mika Penttilä >>>>>>>>>>>>>>>> Cc: Matthew Brost >>>>>>>>>>>>>>>> Cc: Francois Dugast >>>>>>>>>>>>>>>> Cc: Ralph Campbell >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Signed-off-by: Matthew Brost >>>>>>>>>>>>>>>> Signed-off-by: Balbir Singh >>>>>>>>>>>>>>>> --- >>>>>>>>>>>>>>>> include/linux/huge_mm.h | 1 + >>>>>>>>>>>>>>>> include/linux/rmap.h | 2 + >>>>>>>>>>>>>>>> include/linux/swapops.h | 17 +++ >>>>>>>>>>>>>>>> mm/huge_memory.c | 268 +++++++++++++++++++++++++++++++++------- >>>>>>>>>>>>>>>> mm/page_vma_mapped.c | 13 +- >>>>>>>>>>>>>>>> mm/pgtable-generic.c | 6 + >>>>>>>>>>>>>>>> mm/rmap.c | 22 +++- >>>>>>>>>>>>>>>> 7 files changed, 278 insertions(+), 51 deletions(-) >>>>>>>>>>>>>>>> >>>>>>>>>>>>>> >>>>>>>>>>>>>> >>>>>>>>>>>>>>>> +/** >>>>>>>>>>>>>>>> + * split_huge_device_private_folio - split a huge device private folio into >>>>>>>>>>>>>>>> + * smaller pages (of order 0), currently used by migrate_device logic to >>>>>>>>>>>>>>>> + * split folios for pages that are partially mapped >>>>>>>>>>>>>>>> + * >>>>>>>>>>>>>>>> + * @folio: the folio to split >>>>>>>>>>>>>>>> + * >>>>>>>>>>>>>>>> + * The caller has to hold the folio_lock and a reference via folio_get >>>>>>>>>>>>>>>> + */ >>>>>>>>>>>>>>>> +int split_device_private_folio(struct folio *folio) >>>>>>>>>>>>>>>> +{ >>>>>>>>>>>>>>>> + struct folio *end_folio = folio_next(folio); >>>>>>>>>>>>>>>> + struct folio *new_folio; >>>>>>>>>>>>>>>> + int ret = 0; >>>>>>>>>>>>>>>> + >>>>>>>>>>>>>>>> + /* >>>>>>>>>>>>>>>> + * Split the folio now. In the case of device >>>>>>>>>>>>>>>> + * private pages, this path is executed when >>>>>>>>>>>>>>>> + * the pmd is split and since freeze is not true >>>>>>>>>>>>>>>> + * it is likely the folio will be deferred_split. >>>>>>>>>>>>>>>> + * >>>>>>>>>>>>>>>> + * With device private pages, deferred splits of >>>>>>>>>>>>>>>> + * folios should be handled here to prevent partial >>>>>>>>>>>>>>>> + * unmaps from causing issues later on in migration >>>>>>>>>>>>>>>> + * and fault handling flows. >>>>>>>>>>>>>>>> + */ >>>>>>>>>>>>>>>> + folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio)); >>>>>>>>>>>>>>> Why can't this freeze fail? The folio is still mapped afaics, why can't there be other references in addition to the caller? >>>>>>>>>>>>>> Based on my off-list conversation with Balbir, the folio is unmapped in >>>>>>>>>>>>>> CPU side but mapped in the device. folio_ref_freeeze() is not aware of >>>>>>>>>>>>>> device side mapping. >>>>>>>>>>>>> Maybe we should make it aware of device private mapping? So that the >>>>>>>>>>>>> process mirrors CPU side folio split: 1) unmap device private mapping, >>>>>>>>>>>>> 2) freeze device private folio, 3) split unmapped folio, 4) unfreeze, >>>>>>>>>>>>> 5) remap device private mapping. >>>>>>>>>>>> Ah ok this was about device private page obviously here, nevermind.. >>>>>>>>>>> Still, isn't this reachable from split_huge_pmd() paths and folio is mapped to CPU page tables as a huge device page by one or more task? >>>>>>>>>> The folio only has migration entries pointing to it. From CPU perspective, >>>>>>>>>> it is not mapped. The unmap_folio() used by __folio_split() unmaps a to-be-split >>>>>>>>>> folio by replacing existing page table entries with migration entries >>>>>>>>>> and after that the folio is regarded as “unmapped”. >>>>>>>>>> >>>>>>>>>> The migration entry is an invalid CPU page table entry, so it is not a CPU >>>>>>>>> split_device_private_folio() is called for device private entry, not migrate entry afaics. >>>>>>>> Yes, but from CPU perspective, both device private entry and migration entry >>>>>>>> are invalid CPU page table entries, so the device private folio is “unmapped” >>>>>>>> at CPU side. >>>>>>> Yes both are "swap entries" but there's difference, the device private ones contribute to mapcount and refcount. >>>>>> Right. That confused me when I was talking to Balbir and looking at v1. >>>>>> When a device private folio is processed in __folio_split(), Balbir needed to >>>>>> add code to skip CPU mapping handling code. Basically device private folios are >>>>>> CPU unmapped and device mapped. >>>>>> >>>>>> Here are my questions on device private folios: >>>>>> 1. How is mapcount used for device private folios? Why is it needed from CPU >>>>>> perspective? Can it be stored in a device private specific data structure? >>>>> >>>>> Mostly like for normal folios, for instance rmap when doing migrate. I think it would make >>>>> common code more messy if not done that way but sure possible. >>>>> And not consuming pfns (address space) at all would have benefits. >>>>> >>>>>> 2. When a device private folio is mapped on device, can someone other than >>>>>> the device driver manipulate it assuming core-mm just skips device private >>>>>> folios (barring the CPU access fault handling)? >>>>>> >>>>>> Where I am going is that can device private folios be treated as unmapped folios >>>>>> by CPU and only device driver manipulates their mappings? >>>>>> >>>>> Yes not present by CPU but mm has bookkeeping on them. The private page has no content >>>>> someone could change while in device, it's just pfn. >>>> >>>> Just to clarify: a device-private entry, like a device-exclusive entry, is a *page table mapping* tracked through the rmap -- even though they are not present page table entries. >>>> >>>> It would be better if they would be present page table entries that are PROT_NONE, but it's tricky to mark them as being "special" device-private, device-exclusive etc. Maybe there are ways to do that in the future. >>>> >>>> Maybe device-private could just be PROT_NONE, because we can identify the entry type based on the folio. device-exclusive is harder ... >>>> >>>> >>>> So consider device-private entries just like PROT_NONE present page table entries. Refcount and mapcount is adjusted accordingly by rmap functions. >>> >>> Thanks for the clarification. >>> >>> So folio_mapcount() for device private folios should be treated the same >>> as normal folios, even if the corresponding PTEs are not accessible from CPUs. >>> Then I wonder if the device private large folio split should go through >>> __folio_split(), the same as normal folios: unmap, freeze, split, unfreeze, >>> remap. Otherwise, how can we prevent rmap changes during the split? >> >> That is what I would expect: Replace device-private by migration entries, perform the migration/split/whatever, restore migration entries to device-private entries. >> >> That will drive the mapcount to 0. > > Great. That matches my expectations as well. One potential optimization could > be since device private entry is already CPU inaccessible TLB flush can be > avoided. Right, I would assume that is already done, or could easily be added. Not using proper migration entries sounds like a hack that we shouldn't start with. We should start with as little special cases as possible in core-mm. For example, as you probably implied, there is nothing stopping concurrent fork() or zap to mess with the refcount+mapcount. -- Cheers, David / dhildenb