linux-kernel - Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place

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Date:   Fri, 6 Aug 2021 11:07:18 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] mm: migrate: Move the page count validation to the
 proper place

Hi Matthew,

> On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote:
>> We've got the expected count for anonymous page or file page by
>> expected_page_refs() at the beginning of migrate_page_move_mapping(),
>> thus we should move the page count validation a little forward to
>> reduce duplicated code.
> 
> Please add an explanation to the changelog for why it's safe to pull
> this out from under the i_pages lock.

Sure. In folio_migrate_mapping(), we are sure that the migration page 
was isolated from lru list and locked, so I think there are no race to 
get the page count without i_pages lock. Please correct me if I missed 
something else. Thanks.

> 
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>>   mm/migrate.c | 10 ++++------
>>   1 file changed, 4 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 239b238..5559571 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -386,11 +386,10 @@ int folio_migrate_mapping(struct address_space *mapping,
>>   	int expected_count = expected_page_refs(mapping, &folio->page) + extra_count;
>>   	long nr = folio_nr_pages(folio);
>>   
>> -	if (!mapping) {
>> -		/* Anonymous page without mapping */
>> -		if (folio_ref_count(folio) != expected_count)
>> -			return -EAGAIN;
>> +	if (folio_ref_count(folio) != expected_count)
>> +		return -EAGAIN;
>>   
>> +	if (!mapping) {
>>   		/* No turning back from here */
>>   		newfolio->index = folio->index;
>>   		newfolio->mapping = folio->mapping;
>> @@ -404,8 +403,7 @@ int folio_migrate_mapping(struct address_space *mapping,
>>   	newzone = folio_zone(newfolio);
>>   
>>   	xas_lock_irq(&xas);
>> -	if (folio_ref_count(folio) != expected_count ||
>> -	    xas_load(&xas) != folio) {
>> +	if (xas_load(&xas) != folio) {
>>   		xas_unlock_irq(&xas);
>>   		return -EAGAIN;
>>   	}
>> -- 
>> 1.8.3.1
>>
>>