linux-kernel - Re: [PATCH v2] mm/migrate_device: Try to handle swapcache pages

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Message-ID: <87wn0npdnc.fsf@nvidia.com>
Date:   Thu, 01 Jun 2023 22:25:35 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     mpenttil@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH v2] mm/migrate_device: Try to handle swapcache pages


"Huang, Ying" <ying.huang@...el.com> writes:

> mpenttil@...hat.com writes:
>
>> From: Mika Penttilä <mpenttil@...hat.com>
>>
>> Migrating file pages and swapcache pages into device memory is not supported.
>> The decision is done based on page_mapping(). For now, swapcache pages are not migrated.
>>
>> Things can however be improved, for swapcache pages. Try to get rid of the swap cache,
>> and if successful, go ahead as with other anonymous pages.
>>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: John Hubbard <jhubbard@...dia.com>
>> Cc: Ralph Campbell <rcampbell@...dia.com>
>> Cc: Huang Ying <ying.huang@...el.com>
>> Signed-off-by: Mika Penttilä <mpenttil@...hat.com>
>> ---
>>
>> v2:
>>   - use folio_test_anon() (Huang, Ying)
>>
>>  
>>  mm/migrate_device.c | 24 ++++++++++++++++++------
>>  1 file changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index d30c9de60b0d..829bbc526758 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -747,13 +747,25 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>>  
>>  		if (is_device_private_page(newpage) ||
>>  		    is_device_coherent_page(newpage)) {
>> -			/*
>> -			 * For now only support anonymous memory migrating to
>> -			 * device private or coherent memory.
>> -			 */
>> +
>>  			if (mapping) {
>> -				src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
>> -				continue;
>> +				struct folio *folio;
>> +
>> +				folio = page_folio(page);
>> +
>> +				/*
>> +				 * For now only support anonymous memory migrating to
>> +				 * device private or coherent memory.
>> +				 *
>> +				 * Try to get rid of swap cache if possible.
>
> I think we can delete the following 2 lines of comments.  They add
> nothing except what code says already.

They are also a bit confusing, because a private file-backed mapping is
treated as anonymous so folio_test_anon() won't filter those
out. 

I will test this tomorrow but the change looks good so please add:

Reviewed-by: Alistair Popple <apopple@...dia.com>

> Otherwise looks good to me.
>
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
>
>> +				 * page is here file or swapcache page, could be shmem also
>> +				 * folio_test_anon() filters out file and shmem
>> +				 *
>> +				 */
>> +				if (!folio_test_anon(folio) || !folio_free_swap(folio)) {
>> +					src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
>> +					continue;
>> +				}
>>  			}
>>  		} else if (is_zone_device_page(newpage)) {
>>  			/*