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

[Huang, Ying]

Mika Penttilä <mpenttil@xxxxxxxxxx> writes:

> H,
>
>
> On 8.5.2023 5.26, Huang, Ying wrote:
>> mpenttil@xxxxxxxxxx writes:
>> 
>>> From: Mika Penttil <mpenttil@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
>>> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
>>> Signed-off-by: Mika Penttil <mpenttil@xxxxxxxxxx>
>>> ---
>>>   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,

You are right.  folio_free_swap() will call folio_test_swapcache(), and
which will check folio_test_swapbacked().

And, IIUC, we don't need to check shmem too.  Because folio_free_swap()
will not return true for shmem pages.

> but maybe for clarity and not depending on it, could be good
> explicitly check for swapcache before calling folio_free_swap().

Yes.  That will be more clear.  I suggest to use folio_test_anon(), that
will filter out shmem pages too.

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

Best Regards,
Huang, Ying