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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b889ac4dbsm1777116e87.63.2025.08.04.16.26.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Aug 2025 16:26:18 -0700 (PDT) Message-ID: <196f11f8-1661-40d2-b6b7-64958efd8b3b@redhat.com> Date: Tue, 5 Aug 2025 02:26:17 +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> <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> <47BC6D8B-7A78-4F2F-9D16-07D6C88C3661@nvidia.com> <2406521e-f5be-474e-b653-e5ad38a1d7de@redhat.com> <920a4f98-a925-4bd6-ad2e-ae842f2f3d94@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 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. > >>> + /* >>> + * 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. > >>> + folio_put(folio); >>> + addr = start; >>> + goto again; >>> + } >>> + >>> mpfn = migrate_pfn(page_to_pfn(page)) | >>> MIGRATE_PFN_MIGRATE; >>> if (is_writable_device_private_entry(entry)) > Balbir > --Mika