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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b88ca296csm1832935e87.119.2025.08.04.21.24.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Aug 2025 21:24:39 -0700 (PDT) Message-ID: <087e40e6-3b3f-4a02-8270-7e6cfdb56a04@redhat.com> Date: Tue, 5 Aug 2025 07:24:38 +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 Cc: David Hildenbrand , 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> <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> <47BC6D8B-7A78-4F2F-9D16-07D6C88C3661@nvidia.com> <2406521e-f5be-474e-b653-e5ad38a1d7de@redhat.com> <920a4f98-a925-4bd6-ad2e-ae842f2f3d94@redhat.com> <196f11f8-1661-40d2-b6b7-64958efd8b3b@redhat.com> Content-Language: en-US From: =?UTF-8?Q?Mika_Penttil=C3=A4?= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hi, On 8/5/25 07:10, Balbir Singh wrote: > On 8/5/25 09:26, Mika Penttilä wrote: >> Hi, >> >> On 8/5/25 01:46, Balbir Singh wrote: >>> On 8/2/25 22:13, Mika Penttilä wrote: >>>> Hi, >>>> >>>> On 8/2/25 13:37, Balbir Singh wrote: >>>>> FYI: >>>>> >>>>> I have the following patch on top of my series that seems to make it work >>>>> without requiring the helper to split device private folios >>>>> >>>> I think this looks much better! >>>> >>> Thanks! >>> >>>>> Signed-off-by: Balbir Singh >>>>> --- >>>>> include/linux/huge_mm.h | 1 - >>>>> lib/test_hmm.c | 11 +++++- >>>>> mm/huge_memory.c | 76 ++++------------------------------------- >>>>> mm/migrate_device.c | 51 +++++++++++++++++++++++++++ >>>>> 4 files changed, 67 insertions(+), 72 deletions(-) >>>>> >>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >>>>> index 19e7e3b7c2b7..52d8b435950b 100644 >>>>> --- a/include/linux/huge_mm.h >>>>> +++ b/include/linux/huge_mm.h >>>>> @@ -343,7 +343,6 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add >>>>> vm_flags_t vm_flags); >>>>> >>>>> bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins); >>>>> -int split_device_private_folio(struct folio *folio); >>>>> int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list, >>>>> unsigned int new_order, bool unmapped); >>>>> int min_order_for_split(struct folio *folio); >>>>> diff --git a/lib/test_hmm.c b/lib/test_hmm.c >>>>> index 341ae2af44ec..444477785882 100644 >>>>> --- a/lib/test_hmm.c >>>>> +++ b/lib/test_hmm.c >>>>> @@ -1625,13 +1625,22 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf) >>>>> * the mirror but here we use it to hold the page for the simulated >>>>> * device memory and that page holds the pointer to the mirror. >>>>> */ >>>>> - rpage = vmf->page->zone_device_data; >>>>> + rpage = folio_page(page_folio(vmf->page), 0)->zone_device_data; >>>>> dmirror = rpage->zone_device_data; >>>>> >>>>> /* FIXME demonstrate how we can adjust migrate range */ >>>>> order = folio_order(page_folio(vmf->page)); >>>>> nr = 1 << order; >>>>> >>>>> + /* >>>>> + * When folios are partially mapped, we can't rely on the folio >>>>> + * order of vmf->page as the folio might not be fully split yet >>>>> + */ >>>>> + if (vmf->pte) { >>>>> + order = 0; >>>>> + nr = 1; >>>>> + } >>>>> + >>>>> /* >>>>> * Consider a per-cpu cache of src and dst pfns, but with >>>>> * large number of cpus that might not scale well. >>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>>> index 1fc1efa219c8..863393dec1f1 100644 >>>>> --- a/mm/huge_memory.c >>>>> +++ b/mm/huge_memory.c >>>>> @@ -72,10 +72,6 @@ static unsigned long deferred_split_count(struct shrinker *shrink, >>>>> struct shrink_control *sc); >>>>> static unsigned long deferred_split_scan(struct shrinker *shrink, >>>>> struct shrink_control *sc); >>>>> -static int __split_unmapped_folio(struct folio *folio, int new_order, >>>>> - struct page *split_at, struct xa_state *xas, >>>>> - struct address_space *mapping, bool uniform_split); >>>>> - >>>>> static bool split_underused_thp = true; >>>>> >>>>> static atomic_t huge_zero_refcount; >>>>> @@ -2924,51 +2920,6 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma, >>>>> pmd_populate(mm, pmd, pgtable); >>>>> } >>>>> >>>>> -/** >>>>> - * 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)); >>>>> - ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true); >>>>> - VM_WARN_ON(ret); >>>>> - for (new_folio = folio_next(folio); new_folio != end_folio; >>>>> - new_folio = folio_next(new_folio)) { >>>>> - zone_device_private_split_cb(folio, new_folio); >>>>> - folio_ref_unfreeze(new_folio, 1 + folio_expected_ref_count( >>>>> - new_folio)); >>>>> - } >>>>> - >>>>> - /* >>>>> - * Mark the end of the folio split for device private THP >>>>> - * split >>>>> - */ >>>>> - zone_device_private_split_cb(folio, NULL); >>>>> - folio_ref_unfreeze(folio, 1 + folio_expected_ref_count(folio)); >>>>> - return ret; >>>>> -} >>>>> - >>>>> static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >>>>> unsigned long haddr, bool freeze) >>>>> { >>>>> @@ -3064,30 +3015,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >>>>> freeze = false; >>>>> if (!freeze) { >>>>> rmap_t rmap_flags = RMAP_NONE; >>>>> - unsigned long addr = haddr; >>>>> - struct folio *new_folio; >>>>> - struct folio *end_folio = folio_next(folio); >>>>> >>>>> if (anon_exclusive) >>>>> rmap_flags |= RMAP_EXCLUSIVE; >>>>> >>>>> - folio_lock(folio); >>>>> - folio_get(folio); >>>>> - >>>>> - split_device_private_folio(folio); >>>>> - >>>>> - for (new_folio = folio_next(folio); >>>>> - new_folio != end_folio; >>>>> - new_folio = folio_next(new_folio)) { >>>>> - addr += PAGE_SIZE; >>>>> - folio_unlock(new_folio); >>>>> - folio_add_anon_rmap_ptes(new_folio, >>>>> - &new_folio->page, 1, >>>>> - vma, addr, rmap_flags); >>>>> - } >>>>> - folio_unlock(folio); >>>>> - folio_add_anon_rmap_ptes(folio, &folio->page, >>>>> - 1, vma, haddr, rmap_flags); >>>>> + folio_ref_add(folio, HPAGE_PMD_NR - 1); >>>>> + if (anon_exclusive) >>>>> + rmap_flags |= RMAP_EXCLUSIVE; >>>>> + folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR, >>>>> + vma, haddr, rmap_flags); >>>>> } >>>>> } >>>>> >>>>> @@ -4065,7 +4001,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order, >>>>> if (nr_shmem_dropped) >>>>> shmem_uncharge(mapping->host, nr_shmem_dropped); >>>>> >>>>> - if (!ret && is_anon) >>>>> + if (!ret && is_anon && !folio_is_device_private(folio)) >>>>> remap_flags = RMP_USE_SHARED_ZEROPAGE; >>>>> >>>>> remap_page(folio, 1 << order, remap_flags); >>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>>> index 49962ea19109..4264c0290d08 100644 >>>>> --- a/mm/migrate_device.c >>>>> +++ b/mm/migrate_device.c >>>>> @@ -248,6 +248,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>>> * page table entry. Other special swap entries are not >>>>> * migratable, and we ignore regular swapped page. >>>>> */ >>>>> + struct folio *folio; >>>>> + >>>>> entry = pte_to_swp_entry(pte); >>>>> if (!is_device_private_entry(entry)) >>>>> goto next; >>>>> @@ -259,6 +261,55 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>>> pgmap->owner != migrate->pgmap_owner) >>>>> goto next; >>>>> >>>>> + folio = page_folio(page); >>>>> + if (folio_test_large(folio)) { >>>>> + struct folio *new_folio; >>>>> + struct folio *new_fault_folio; >>>>> + >>>>> + /* >>>>> + * The reason for finding pmd present with a >>>>> + * device private pte and a large folio for the >>>>> + * pte is partial unmaps. Split the folio now >>>>> + * for the migration to be handled correctly >>>>> + */ >>>>> + pte_unmap_unlock(ptep, ptl); >>>>> + >>>>> + folio_get(folio); >>>>> + if (folio != fault_folio) >>>>> + folio_lock(folio); >>>>> + if (split_folio(folio)) { >>>>> + if (folio != fault_folio) >>>>> + folio_unlock(folio); >>>>> + ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl); >>>>> + goto next; >>>>> + } >>>>> + >>>> The nouveau migrate_to_ram handler needs adjustment also if split happens. >>>> >>> test_hmm needs adjustment because of the way the backup folios are setup. >> nouveau should check the folio order after the possible split happens. >> > You mean the folio_split callback? no, nouveau_dmem_migrate_to_ram(): .. sfolio = page_folio(vmf->page); order = folio_order(sfolio); ... migrate_vma_setup() .. if sfolio is split order still reflects the pre-split order > >>>>> + /* >>>>> + * After the split, get back the extra reference >>>>> + * on the fault_page, this reference is checked during >>>>> + * folio_migrate_mapping() >>>>> + */ >>>>> + if (migrate->fault_page) { >>>>> + new_fault_folio = page_folio(migrate->fault_page); >>>>> + folio_get(new_fault_folio); >>>>> + } >>>>> + >>>>> + new_folio = page_folio(page); >>>>> + pfn = page_to_pfn(page); >>>>> + >>>>> + /* >>>>> + * Ensure the lock is held on the correct >>>>> + * folio after the split >>>>> + */ >>>>> + if (folio != new_folio) { >>>>> + folio_unlock(folio); >>>>> + folio_lock(new_folio); >>>>> + } >>>> Maybe careful not to unlock fault_page ? >>>> >>> split_page will unlock everything but the original folio, the code takes the lock >>> on the folio corresponding to the new folio >> I mean do_swap_page() unlocks folio of fault_page and expects it to remain locked. >> > Not sure I follow what you're trying to elaborate on here do_swap_page: .. if (trylock_page(vmf->page)) { ret = pgmap->ops->migrate_to_ram(vmf); <- vmf->page should be locked here even after split unlock_page(vmf->page); > Balbir > --Mika