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Message-ID: <d4ae7f61-f2f9-45dc-91db-4de7f3127744@redhat.com>
Date: Thu, 11 Sep 2025 14:42:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
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: [v5 07/15] mm/memory/fault: add THP fault handling for zone
device private pages
On 08.09.25 02:04, Balbir Singh wrote:
> Implement CPU fault handling for zone device THP entries through
> do_huge_pmd_device_private(), enabling transparent migration of
> device-private large pages back to system memory on CPU access.
>
> When the CPU accesses a zone device THP entry, the fault handler
> calls the device driver's migrate_to_ram() callback to migrate
> the entire large page back to system memory.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> 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>
>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
> include/linux/huge_mm.h | 7 +++++++
> mm/huge_memory.c | 36 ++++++++++++++++++++++++++++++++++++
> mm/memory.c | 6 ++++--
> 3 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 23f124493c47..2c6a0c3c862c 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -496,6 +496,8 @@ static inline bool folio_test_pmd_mappable(struct folio *folio)
>
> vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf);
>
> +vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf);
> +
> extern struct folio *huge_zero_folio;
> extern unsigned long huge_zero_pfn;
>
> @@ -675,6 +677,11 @@ static inline vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
> return 0;
> }
>
> +static inline vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
> +{
Is this a VM_WARN_ON_ONCE() or similar? (Maybe BUILD_BUG is possible?)
> + return 0;
> +}
> +
> static inline bool is_huge_zero_folio(const struct folio *folio)
> {
> return false;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b720870c04b2..d634b2157a56 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1287,6 +1287,42 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>
> }
>
> +vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + vm_fault_t ret = 0;
> + spinlock_t *ptl;
> + swp_entry_t swp_entry;
> + struct page *page;
> +
> + if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> + vma_end_read(vma);
> + return VM_FAULT_RETRY;
> + }
> +
> + ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> + if (unlikely(!pmd_same(*vmf->pmd, vmf->orig_pmd))) {
> + spin_unlock(ptl);
> + return 0;
> + }
> +
> + swp_entry = pmd_to_swp_entry(vmf->orig_pmd);
> + page = pfn_swap_entry_to_page(swp_entry);
> + vmf->page = page;
> + vmf->pte = NULL;
> + if (trylock_page(vmf->page)) {
> + get_page(page);
> + spin_unlock(ptl);
> + ret = page_pgmap(page)->ops->migrate_to_ram(vmf);
> + unlock_page(vmf->page);
> + put_page(page);
> + } else {
> + spin_unlock(ptl);
> + }
> +
> + return ret;
> +}
> +
> /*
> * always: directly stall for all thp allocations
> * defer: wake kswapd and fail if not immediately available
> diff --git a/mm/memory.c b/mm/memory.c
> index d9de6c056179..860665f4b692 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6298,8 +6298,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
> vmf.orig_pmd = pmdp_get_lockless(vmf.pmd);
>
> if (unlikely(is_swap_pmd(vmf.orig_pmd))) {
> - VM_BUG_ON(thp_migration_supported() &&
> - !is_pmd_migration_entry(vmf.orig_pmd));
> + if (is_device_private_entry(
> + pmd_to_swp_entry(vmf.orig_pmd)))
Single line please.
But didn't we have a pmd helper for that?
> + return do_huge_pmd_device_private(&vmf);
> +
> if (is_pmd_migration_entry(vmf.orig_pmd))
> pmd_migration_entry_wait(mm, vmf.pmd);
> return 0;
--
Cheers
David / dhildenb
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