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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55b88cabf55sm1927641e87.147.2025.08.05.03.35.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 05 Aug 2025 03:35:12 -0700 (PDT) Message-ID: <6a08fa8f-bc39-4389-aa52-d95f82538a91@redhat.com> Date: Tue, 5 Aug 2025 13:35:11 +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> <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> <087e40e6-3b3f-4a02-8270-7e6cfdb56a04@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 On 8/5/25 13:27, Balbir Singh wrote: > On 8/5/25 14:24, Mika Penttilä wrote: >> 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 >> > Will fix, good catch! > >>>>>>> + /* >>>>>>> + * 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); >> > Yep, the split will unlock all tail folios, leaving the just head folio locked > and this the change, the lock we need to hold is the folio lock associated with > fault_page, pte entry and not unlock when the cause is a fault. The code seems > to do the right thing there, let me double check Yes the fault case is ok. But if migrate not for a fault, we should not leave any page locked > > Balbir > and the code does the right thing there. > --Mika