Re: [PATCH] mm/migrate: fix shmem xarray update during migration

[Zi Yan]

On 28 Feb 2025, at 12:02, Shivank Garg wrote:

> On 2/28/2025 9:12 PM, Zi Yan wrote:
>> Pagecache uses multi-index entries for large folio, so does shmem. Only
>> swap cache still stores multiple entries for a single large folio.
>> Commit fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
>> fixed swap cache but got shmem wrong by storing multiple entries for
>> a large shmem folio. Fix it by storing a single entry for a shmem
>> folio.
>>
>> Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
>> Reported-by: Liu Shixin <liushixin2@xxxxxxxxxx>
>> Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@xxxxxxxxxx/
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>  mm/migrate.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 365c6daa8d1b..9db26f5527a8 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -44,6 +44,7 @@
>>  #include <linux/sched/sysctl.h>
>>  #include <linux/memory-tiers.h>
>>  #include <linux/pagewalk.h>
>> +#include <linux/shmem_fs.h>
>>
>>  #include <asm/tlbflush.h>
>>
>> @@ -524,7 +525,11 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>>  			folio_set_swapcache(newfolio);
>>  			newfolio->private = folio_get_private(folio);
>>  		}
>> -		entries = nr;
>> +		/* shmem uses high-order entry */
>> +		if (shmem_mapping(mapping))
>> +			entries = 1;
>> +		else
>> +			entries = nr;
>
> LGTM functionally.
> As a minor style suggestion, can we consider using a ternary operator:
>
> 	entries = shmem_mapping(mapping) ? 1 : nr;  /* shmem uses high-order entry */
>
> This looks cleaner to me.
>
> Reviewed-by: Shivank Garg <shivankg@xxxxxxx>

Thanks.

Best Regards,
Yan, Zi