linux-kernel - Re: [v7 11/16] mm/migrate_device: add THP splitting during migration

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Message-ID: <62073ca1-5bb6-49e8-b8d4-447c5e0e582e@nvidia.com>
Date: Mon, 20 Oct 2025 09:49:03 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-mm@...ck.org, akpm@...ux-foundation.org,
 David Hildenbrand <david@...hat.com>, 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: [v7 11/16] mm/migrate_device: add THP splitting during migration

On 10/19/25 19:19, Wei Yang wrote:
> On Wed, Oct 01, 2025 at 04:57:02PM +1000, Balbir Singh wrote:
> [...]
>> static int __folio_split(struct folio *folio, unsigned int new_order,
>> 		struct page *split_at, struct page *lock_at,
>> -		struct list_head *list, bool uniform_split)
>> +		struct list_head *list, bool uniform_split, bool unmapped)
>> {
>> 	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>> 	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>> @@ -3765,13 +3757,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>> 		 * is taken to serialise against parallel split or collapse
>> 		 * operations.
>> 		 */
>> -		anon_vma = folio_get_anon_vma(folio);
>> -		if (!anon_vma) {
>> -			ret = -EBUSY;
>> -			goto out;
>> +		if (!unmapped) {
>> +			anon_vma = folio_get_anon_vma(folio);
>> +			if (!anon_vma) {
>> +				ret = -EBUSY;
>> +				goto out;
>> +			}
>> +			anon_vma_lock_write(anon_vma);
>> 		}
>> 		mapping = NULL;
>> -		anon_vma_lock_write(anon_vma);
>> 	} else {
>> 		unsigned int min_order;
>> 		gfp_t gfp;
>> @@ -3838,7 +3832,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>> 		goto out_unlock;
>> 	}
>>
>> -	unmap_folio(folio);
>> +	if (!unmapped)
>> +		unmap_folio(folio);
>>
>> 	/* block interrupt reentry in xa_lock and spinlock */
>> 	local_irq_disable();
>> @@ -3925,10 +3920,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>
>> 			next = folio_next(new_folio);
>>
>> +			zone_device_private_split_cb(folio, new_folio);
>> +
>> 			expected_refs = folio_expected_ref_count(new_folio) + 1;
>> 			folio_ref_unfreeze(new_folio, expected_refs);
>>
>> -			lru_add_split_folio(folio, new_folio, lruvec, list);
>> +			if (!unmapped)
>> +				lru_add_split_folio(folio, new_folio, lruvec, list);
>>
>> 			/*
>> 			 * Anonymous folio with swap cache.
>> @@ -3959,6 +3957,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>> 			__filemap_remove_folio(new_folio, NULL);
>> 			folio_put_refs(new_folio, nr_pages);
>> 		}
>> +
>> +		zone_device_private_split_cb(folio, NULL);
>> 		/*
>> 		 * Unfreeze @folio only after all page cache entries, which
>> 		 * used to point to it, have been updated with new folios.
>> @@ -3982,6 +3982,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>
>> 	local_irq_enable();
>>
>> +	if (unmapped)
>> +		return ret;
> 
> As the comment of __folio_split() and __split_huge_page_to_list_to_order()
> mentioned:
> 
>   * The large folio must be locked
>   * After splitting, the after-split folio containing @lock_at remains locked
> 
> But here we seems to change the prerequisites.
> 
> Hmm.. I am not sure this is correct.
> 

The code is correct, but you are right in that the documentation needs to be updated.
When "unmapped", we do want to leave the folios locked after the split.

Balbir