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Message-ID: <4fa4f448-ebef-47dd-ba99-6ef6e5862fda@redhat.com>
Date: Tue, 5 Aug 2025 08:19:27 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Balbir Singh <balbirs@...dia.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 07:24, 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 <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?
> 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
>
Actually fault_folio should be fine but should we have:
if (fault_folio)
if(folio != new_folio)) {
folio_unlock(folio);
folio_lock(new_folio);
}
else
folio_unlock(folio);
>> Balbir
>>
> --Mika
>
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