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Message-ID: <920a4f98-a925-4bd6-ad2e-ae842f2f3d94@redhat.com>
Date: Sat, 2 Aug 2025 15:13:33 +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
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!
> 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.
> + /*
> + * 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 ?
> + folio_put(folio);
> + addr = start;
> + goto again;
> + }
> +
> mpfn = migrate_pfn(page_to_pfn(page)) |
> MIGRATE_PFN_MIGRATE;
> if (is_writable_device_private_entry(entry))
--Mika
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