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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b63393c72sm2127002e87.178.2025.07.30.05.08.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 30 Jul 2025 05:08:48 -0700 (PDT) Message-ID: <884b9246-de7c-4536-821f-1bf35efe31c8@redhat.com> Date: Wed, 30 Jul 2025 15:08:47 +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 From: =?UTF-8?Q?Mika_Penttil=C3=A4?= To: Zi Yan Cc: 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 , David Hildenbrand , 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> Content-Language: en-US In-Reply-To: <9f836828-4f53-41a0-b5f7-bbcd2084086e@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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? > >>>>> + ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true); >>>> Confusing to  __split_unmapped_folio() if folio is mapped... >>> From driver point of view, __split_unmapped_folio() probably should be renamed >>> to __split_cpu_unmapped_folio(), since it is only dealing with CPU side >>> folio meta data for split. >>> >>> >>> Best Regards, >>> Yan, Zi >> Best Regards, >> Yan, Zi >> --Mika