linux-kernel - Re: [v1 resend 08/12] mm/thp: add split during migration support

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Message-ID: <1a451d37-56c3-4d60-8e06-3abae72e6bbd@nvidia.com>
Date: Sat, 5 Jul 2025 10:58:24 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.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>, David Hildenbrand <david@...hat.com>,
 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>
Subject: Re: [v1 resend 08/12] mm/thp: add split during migration support

On 7/4/25 21:24, Zi Yan wrote:
> 
> s/pages/folio
> 

Thanks, will make the changes

> Why name it isolated if the folio is unmapped? Isolated folios often mean
> they are removed from LRU lists. isolated here causes confusion.
> 

Ack, will change the name


>>   *
>>   * It calls __split_unmapped_folio() to perform uniform and non-uniform split.
>>   * It is in charge of checking whether the split is supported or not and
>> @@ -3800,7 +3799,7 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>>   */
>>  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 isolated)
>>  {
>>  	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>  	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>> @@ -3846,14 +3845,16 @@ 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 (!isolated) {
>> +			anon_vma = folio_get_anon_vma(folio);
>> +			if (!anon_vma) {
>> +				ret = -EBUSY;
>> +				goto out;
>> +			}
>> +			anon_vma_lock_write(anon_vma);
>>  		}
>>  		end = -1;
>>  		mapping = NULL;
>> -		anon_vma_lock_write(anon_vma);
>>  	} else {
>>  		unsigned int min_order;
>>  		gfp_t gfp;
>> @@ -3920,7 +3921,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>  		goto out_unlock;
>>  	}
>>
>> -	unmap_folio(folio);
>> +	if (!isolated)
>> +		unmap_folio(folio);
>>
>>  	/* block interrupt reentry in xa_lock and spinlock */
>>  	local_irq_disable();
>> @@ -3973,14 +3975,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>
>>  		ret = __split_unmapped_folio(folio, new_order,
>>  				split_at, lock_at, list, end, &xas, mapping,
>> -				uniform_split);
>> +				uniform_split, isolated);
>>  	} else {
>>  		spin_unlock(&ds_queue->split_queue_lock);
>>  fail:
>>  		if (mapping)
>>  			xas_unlock(&xas);
>>  		local_irq_enable();
>> -		remap_page(folio, folio_nr_pages(folio), 0);
>> +		if (!isolated)
>> +			remap_page(folio, folio_nr_pages(folio), 0);
>>  		ret = -EAGAIN;
>>  	}
> 
> These "isolated" special handlings does not look good, I wonder if there
> is a way of letting split code handle device private folios more gracefully.
> It also causes confusions, since why does "isolated/unmapped" folios
> not need to unmap_page(), remap_page(), or unlock?
> 
> 

There are two reasons for going down the current code path

1. if the isolated check is not present, folio_get_anon_vma will fail and cause
   the split routine to return with -EBUSY
2. Going through unmap_page(), remap_page() causes a full page table walk, which
   the migrate_device API has already just done as a part of the migration. The
   entries under consideration are already migration entries in this case.
   This is wasteful and in some case unexpected.


Thanks for the review,
Balbir Singh