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Message-ID: <ede98867-bc17-4894-a375-d17d8f83dd55@nvidia.com>
Date: Tue, 5 Aug 2025 14:10:42 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Mika Penttilä <mpenttil@...hat.com>,
Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Karol Herbst <kherbst@...hat.com>,
Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
Jérôme Glisse <jglisse@...hat.com>,
Shuah Khan <shuah@...nel.org>, Barry Song <baohua@...nel.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>,
Peter Xu <peterx@...hat.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
Jane Chu <jane.chu@...cle.com>, Alistair Popple <apopple@...dia.com>,
Donet Tom <donettom@...ux.ibm.com>, Matthew Brost <matthew.brost@...el.com>,
Francois Dugast <francois.dugast@...el.com>,
Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code
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 <balbirs@...dia.com>
>>>> ---
>>>> 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?
>>
>>>> + /*
>>>> + * 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
Balbir
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