linux-kernel - Re: [PATCH 2/4] mm: migrate: convert numamigrate_isolate_page() to numamigrate_isolate

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Date:   Thu, 3 Aug 2023 15:08:10 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Huang Ying <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 2/4] mm: migrate: convert numamigrate_isolate_page() to
 numamigrate_isolate_folio()



On 2023/8/2 20:30, Matthew Wilcox wrote:
> On Wed, Aug 02, 2023 at 05:53:44PM +0800, Kefeng Wang wrote:
>> -static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
>> +static int numamigrate_isolate_folio(pg_data_t *pgdat, struct folio *folio)
>>   {
>> -	int nr_pages = thp_nr_pages(page);
>> -	int order = compound_order(page);
>> +	int nr_pages = folio_nr_pages(folio);
>> +	int order = folio_order(folio);
>>   
>> -	VM_BUG_ON_PAGE(order && !PageTransHuge(page), page);
>> +	VM_BUG_ON_FOLIO(order && !folio_test_pmd_mappable(folio), folio);
> 
> I don't know why we have this assertion.  I would be inclined to delete
> it as part of generalising the migration code to handle arbitrary sizes
> of folio, rather than assert that we only support PMD size folios.

Ok, will drop it.
> 
>>   	/* Do not migrate THP mapped by multiple processes */
>> -	if (PageTransHuge(page) && total_mapcount(page) > 1)
>> +	if (folio_test_pmd_mappable(folio) && folio_estimated_sharers(folio) > 1)
>>   		return 0;
> 
> I don't know if this is the right logic.  We've willing to move folios
> mapped by multiple processes, as long as they're smaller than PMD size,
> but once they get to PMD size they're magical and can't be moved?

It seems that the logical is introduced by commit 04fa5d6a6547 ("mm:
migrate: check page_count of THP before migrating") and refactor by
340ef3902cf2 ("mm: numa: cleanup flow of transhuge page migration"),


   "Hugh Dickins pointed out that migrate_misplaced_transhuge_page() does
   not check page_count before migrating like base page migration and
   khugepage.  He could not see why this was safe and he is right."

For now, there is no migrate_misplaced_transhuge_page() and base/thp
page migrate's path is unified, there is a check(for old/new kernel) in 
migrate_misplaced_page(),

  "Don't migrate file pages that are mapped in multiple processes
  with execute permissions as they are probably shared libraries."

We could drop the above check in numamigrate_isolate_page(), but
according to 04fa5d6a6547, maybe disable migrate page shared by
multi-process during numa balance for both base/thp page.


> 
>