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Message-ID: <7987AB89-4E80-4A0D-8736-E95F998698DA@nvidia.com>
Date: Mon, 22 Sep 2025 17:09:16 -0400
From: Zi Yan <ziy@...dia.com>
To: Balbir Singh <balbirs@...dia.com>, David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, damon@...ts.linux.dev,
dri-devel@...ts.freedesktop.org, Joshua Hahn <joshua.hahnjy@...il.com>,
Rakie Kim <rakie.kim@...com>, Byungchul Park <byungchul@...com>,
Gregory Price <gourry@...rry.net>, Ying Huang <ying.huang@...ux.alibaba.com>,
Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
Mika Penttilä <mpenttil@...hat.com>,
Matthew Brost <matthew.brost@...el.com>,
Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v6 04/15] mm/huge_memory: implement device-private THP splitting
On 16 Sep 2025, at 8:21, Balbir Singh wrote:
> Add support for splitting device-private THP folios, enabling fallback
> to smaller page sizes when large page allocation or migration fails.
>
> Key changes:
> - split_huge_pmd(): Handle device-private PMD entries during splitting
> - Preserve RMAP_EXCLUSIVE semantics for anonymous exclusive folios
> - Skip RMP_USE_SHARED_ZEROPAGE for device-private entries as they
> don't support shared zero page semantics
>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
> Cc: Rakie Kim <rakie.kim@...com>
> Cc: Byungchul Park <byungchul@...com>
> Cc: Gregory Price <gourry@...rry.net>
> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Matthew Brost <matthew.brost@...el.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
> ---
> mm/huge_memory.c | 138 +++++++++++++++++++++++++++++++++--------------
> 1 file changed, 98 insertions(+), 40 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 78166db72f4d..5291ee155a02 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2872,16 +2872,18 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> struct page *page;
> pgtable_t pgtable;
> pmd_t old_pmd, _pmd;
> - bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
> - bool anon_exclusive = false, dirty = false;
> + bool soft_dirty, uffd_wp = false, young = false, write = false;
> + bool anon_exclusive = false, dirty = false, present = false;
> unsigned long addr;
> pte_t *pte;
> int i;
> + swp_entry_t swp_entry;
>
> VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
> VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
> VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
> - VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd));
> +
> + VM_WARN_ON(!is_pmd_non_present_folio_entry(*pmd) && !pmd_trans_huge(*pmd));
>
> count_vm_event(THP_SPLIT_PMD);
>
> @@ -2929,20 +2931,47 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> return __split_huge_zero_page_pmd(vma, haddr, pmd);
> }
>
> - pmd_migration = is_pmd_migration_entry(*pmd);
> - if (unlikely(pmd_migration)) {
> - swp_entry_t entry;
>
> + present = pmd_present(*pmd);
> + if (is_pmd_migration_entry(*pmd)) {
> old_pmd = *pmd;
> - entry = pmd_to_swp_entry(old_pmd);
> - page = pfn_swap_entry_to_page(entry);
> - write = is_writable_migration_entry(entry);
> + swp_entry = pmd_to_swp_entry(old_pmd);
> + page = pfn_swap_entry_to_page(swp_entry);
> + folio = page_folio(page);
> +
> + soft_dirty = pmd_swp_soft_dirty(old_pmd);
> + uffd_wp = pmd_swp_uffd_wp(old_pmd);
> +
> + write = is_writable_migration_entry(swp_entry);
> if (PageAnon(page))
> - anon_exclusive = is_readable_exclusive_migration_entry(entry);
> - young = is_migration_entry_young(entry);
> - dirty = is_migration_entry_dirty(entry);
> + anon_exclusive = is_readable_exclusive_migration_entry(swp_entry);
> + young = is_migration_entry_young(swp_entry);
> + dirty = is_migration_entry_dirty(swp_entry);
> + } else if (is_pmd_device_private_entry(*pmd)) {
> + old_pmd = *pmd;
> + swp_entry = pmd_to_swp_entry(old_pmd);
> + page = pfn_swap_entry_to_page(swp_entry);
> + folio = page_folio(page);
> +
> soft_dirty = pmd_swp_soft_dirty(old_pmd);
> uffd_wp = pmd_swp_uffd_wp(old_pmd);
> +
> + write = is_writable_device_private_entry(swp_entry);
> + anon_exclusive = PageAnonExclusive(page);
> +
> + if (freeze && anon_exclusive &&
> + folio_try_share_anon_rmap_pmd(folio, page))
> + freeze = false;
Why is it OK to change the freeze request? OK, it is replicating
the code for present PMD folios. Either add a comment to point
to the explanation in the comment below, or move
“if (is_pmd_device_private_entry(*pmd))“ branch in the else below
to deduplicate this code.
> + if (!freeze) {
> + rmap_t rmap_flags = RMAP_NONE;
> +
> + 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);
> + }
> } else {
> /*
> * Up to this point the pmd is present and huge and userland has
> @@ -3026,32 +3055,57 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> * Note that NUMA hinting access restrictions are not transferred to
> * avoid any possibility of altering permissions across VMAs.
> */
> - if (freeze || pmd_migration) {
> - for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> - pte_t entry;
> - swp_entry_t swp_entry;
> -
> - if (write)
> - swp_entry = make_writable_migration_entry(
> - page_to_pfn(page + i));
> - else if (anon_exclusive)
> - swp_entry = make_readable_exclusive_migration_entry(
> - page_to_pfn(page + i));
> - else
> - swp_entry = make_readable_migration_entry(
> - page_to_pfn(page + i));
> - if (young)
> - swp_entry = make_migration_entry_young(swp_entry);
> - if (dirty)
> - swp_entry = make_migration_entry_dirty(swp_entry);
> - entry = swp_entry_to_pte(swp_entry);
> - if (soft_dirty)
> - entry = pte_swp_mksoft_dirty(entry);
> - if (uffd_wp)
> - entry = pte_swp_mkuffd_wp(entry);
> + if (freeze || !present) {
> + pte_t entry;
>
> - VM_WARN_ON(!pte_none(ptep_get(pte + i)));
> - set_pte_at(mm, addr, pte + i, entry);
> + if (freeze || is_migration_entry(swp_entry)) {
>
<snip>
> + } else {
<snip>
> }
> } else {
> pte_t entry;
David already pointed this out in v5. It can be done such as:
if (freeze || pmd_migration) {
...
} else if (is_pmd_device_private_entry(old_pmd)) {
...
} else {
/* for present, non freeze case */
}
> @@ -3076,7 +3130,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> }
> pte_unmap(pte);
>
> - if (!pmd_migration)
> + if (!is_pmd_migration_entry(*pmd))
> folio_remove_rmap_pmd(folio, page, vma);
> if (freeze)
> put_page(page);
> @@ -3089,7 +3143,7 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> pmd_t *pmd, bool freeze)
> {
> VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
> - if (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd))
> + if (pmd_trans_huge(*pmd) || is_pmd_non_present_folio_entry(*pmd))
> __split_huge_pmd_locked(vma, pmd, address, freeze);
> }
>
> @@ -3268,6 +3322,9 @@ static void lru_add_split_folio(struct folio *folio, struct folio *new_folio,
> VM_BUG_ON_FOLIO(folio_test_lru(new_folio), folio);
> lockdep_assert_held(&lruvec->lru_lock);
>
> + if (folio_is_device_private(folio))
> + return;
> +
> if (list) {
> /* page reclaim is reclaiming a huge page */
> VM_WARN_ON(folio_test_lru(folio));
> @@ -3885,8 +3942,9 @@ 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;
> +
You should remove this and add
if (folio_is_device_private(folio))
return false;
in try_to_map_unused_to_zeropage(). Otherwise, no one would know
device private folios need to be excluded from mapping unused to
zero page.
> remap_page(folio, 1 << order, remap_flags);
>
> /*
> --
> 2.50.1
Best Regards,
Yan, Zi
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