linux-kernel - Re: [v6 07/15] mm/memory/fault: add THP fault handling for zone device private pages

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Message-ID: <4d8142b8-a187-4519-a174-7b3bc594b4df@nvidia.com>
Date: Tue, 30 Sep 2025 22:00:16 +1000
From: Balbir Singh <balbirs@...dia.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Alistair Popple <apopple@...dia.com>
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.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>, 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 07/15] mm/memory/fault: add THP fault handling for zone
 device private pages

On 9/25/25 20:11, David Hildenbrand wrote:
> On 16.09.25 14:21, 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.
>>
>> 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>
>> ---
>>   include/linux/huge_mm.h |  7 +++++++
>>   mm/huge_memory.c        | 36 ++++++++++++++++++++++++++++++++++++
>>   mm/memory.c             |  5 +++--
>>   3 files changed, 46 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index f327d62fc985..2d669be7f1c8 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;
>>   @@ -671,6 +673,11 @@ 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)
>> +{
>> +    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 5291ee155a02..90a1939455dd 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)) {
> 
> We should be operating on a folio here. folio_trylock() + folio_get() + folio_unlock() + folio_put().
> 
>> +        get_page(page);
>> +        spin_unlock(ptl);
>> +        ret = page_pgmap(page)->ops->migrate_to_ram(vmf);
> 
> BTW, I was wondering whether it is really the right design to pass the vmf here. Likely the const vma+addr+folio could be sufficient. I did not look into all callbaks, though.
> 

The vmf is used for address and other bits. FYI, this is no different from pte fault handling and migrate_to_ram(). I can do the folio conversions

Balbir