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

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Message-ID: <f93c14f6-024d-4abc-7598-fa82cc3ea1a5@redhat.com>
Date:   Mon, 8 May 2023 05:59:46 +0300
From:   Mika Penttilä <mpenttil@...hat.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        apopple@...dia.com, jhubbard@...dia.com, rcampbell@...dia.com
Subject: Re: [PATCH] mm/migrate_device: Try to handle swapcache pages.

H,


On 8.5.2023 5.26, Huang, Ying wrote:
> 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.
>>
>> As a detail, do not mess with shmem pages, as they handle swap internally.
>>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: John Hubbard <jhubbard@...dia.com>
>> Cc: Ralph Campbell <rcampbell@...dia.com>
>> Signed-off-by: Mika Penttilä <mpenttil@...hat.com>
>> ---
>>   mm/migrate_device.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index d30c9de60b0d..e8169c58c341 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -12,6 +12,7 @@
>>   #include <linux/mmu_notifier.h>
>>   #include <linux/oom.h>
>>   #include <linux/pagewalk.h>
>> +#include <linux/shmem_fs.h>
>>   #include <linux/rmap.h>
>>   #include <linux/swapops.h>
>>   #include <asm/tlbflush.h>
>> @@ -750,10 +751,17 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>>   			/*
>>   			 * For now only support anonymous memory migrating to
>>   			 * device private or coherent memory.
>> +			 *
>> +			 * Try to get rid of swap cache if possible.
>> +			 * Leave shmem pages alone, they handle swap internally
>> +			 *
>>   			 */
>>   			if (mapping) {
>> -				src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
>> -				continue;
>> +				if (shmem_mapping(mapping) ||
>> +				    !folio_free_swap(page_folio(page))) {
> 
> Should we filter out file pages firstly?

folio_free_swap() checks first and returns false if not swapcache page 
(ie including normal file pages) so think that is enough, but maybe for 
clarity and not depending on it, could be good explicitly check for 
swapcache before calling folio_free_swap().

> 
>> +					src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
>> +					continue;
>> +				}
>>   			}
>>   		} else if (is_zone_device_page(newpage)) {
>>   			/*
> 
> Best Regards,
> Huang, Ying
> 

Thanks,
Mika