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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b8899dc75sm456787e87.40.2025.07.31.23.01.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 31 Jul 2025 23:01:12 -0700 (PDT) Message-ID: Date: Fri, 1 Aug 2025 09:01:12 +0300 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: Balbir Singh , Zi Yan , David Hildenbrand Cc: 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> <14aeaecc-c394-41bf-ae30-24537eb299d9@nvidia.com> <71c736e9-eb77-4e8e-bd6a-965a1bbcbaa8@nvidia.com> Content-Language: en-US From: =?UTF-8?Q?Mika_Penttil=C3=A4?= In-Reply-To: <71c736e9-eb77-4e8e-bd6a-965a1bbcbaa8@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 8/1/25 07:44, Balbir Singh wrote: > On 8/1/25 11:16, Mika Penttilä wrote: >> Hi, >> >> On 8/1/25 03:49, Balbir Singh wrote: >> >>> On 7/31/25 21:26, 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 true in general, the special cases I mentioned are: >>> >>> 1. split during migration (where we the sizes on source/destination do not >>> match) and so we need to split in the middle of migration. The entries >>> there are already unmapped and hence the special handling >>> 2. Partial unmap case, where we need to split in the context of the unmap >>> due to the isses mentioned in the patch. I expanded the folio split code >>> for device private can be expanded into its own helper, which does not >>> need to do the xas/mapped/lru folio handling. During partial unmap the >>> original folio does get replaced by new anon rmap ptes (split_huge_pmd_locked) >>> >>> For (2), I spent some time examining the implications of not unmapping the >>> folios prior to split and in the partial unmap path, once we split the PMD >>> the folios diverge. I did not run into any particular race either with the >>> tests. >> 1) is totally fine. This was in v1 and lead to Zi's split_unmapped_folio() >> >> 2) is a problem because folio is mapped. split_huge_pmd() can be reached also from other than unmap path. >> It is vulnerable to races by rmap. And for instance this does not look right without checking: >> >> folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio)); >> > I can add checks to make sure that the call does succeed. > >> You mention 2) is needed because of some later problems in fault path after pmd split. Would it be >> possible to split the folio at fault time then? > So after the partial unmap, the folio ends up in a little strange situation, the folio is large, > but not mapped (since large_mapcount can be 0, after all the folio_rmap_remove_ptes). Calling folio_split() > on partially unmapped fails because folio_get_anon_vma() fails due to the folio_mapped() failures > related to folio_large_mapcount. There is also additional complexity with ref counts and mapping. Is this after the deferred split -> map_unused_to_zeropage flow which would leave the page unmapped? Maybe disable that for device pages? > > >> Also, didn't quite follow what kind of lock recursion did you encounter doing proper split_folio() >> instead? >> >> > Splitting during partial unmap causes recursive locking issues with anon_vma when invoked from > split_huge_pmd_locked() path. Deferred splits do not work for device private pages, due to the > migration requirements for fault handling. > > Balbir Singh --Mika