Re: [RFC PATCH v4 2/6] mm/migrate: skip data copy for already-copied folios

[Garg, Shivank]

On 3/24/2026 1:52 PM, Huang, Ying wrote:
> Shivank Garg <shivankg@xxxxxxx> writes:
> 

>>  static int move_to_new_folio(struct folio *dst, struct folio *src,
>> -				enum migrate_mode mode)
>> +		enum migrate_mode mode, bool already_copied)
>>  {
>>  	struct address_space *mapping = folio_mapping(src);
>>  	int rc = -EAGAIN;
>> @@ -1096,6 +1114,9 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>>  	VM_BUG_ON_FOLIO(!folio_test_locked(src), src);
>>  	VM_BUG_ON_FOLIO(!folio_test_locked(dst), dst);
>>  
>> +	if (already_copied)
>> +		dst->private = (void *)(unsigned long)PAGE_ALREADY_COPIED;
>> +
> 
> IMHO, this appears to be an unusual way to pass arguments to a function.
> Why not adjust the parameters of migrate_folio()?  How about turning enum
> migrate_mode into a bitmask (migrate_flags)?
>

Using folio->private, keeps the change self-contained in migrate.c

David suggested adding a dedicated unsigned long migrate_info field in the
folio union. I'll switch to that, as this is cleaner and avoid hacky use of ->private.

https://lore.kernel.org/linux-mm/27b1b602-129f-4bc5-a553-386e8d1f5d90@xxxxxxxxxx


Changing the migrate_folio() a_ops signature would touch nearly every
filesystem for something that only core migration cares about, and does not look
practical. We can't add to migrate_mode enum either as currently those values are mutually
exclusive and ordered levels of increasing synchrony (ASYNC < SYNC_LIGHT < SYNC)
And there are checks like this (cc->mode < MIGRATE_SYNC) or  mode != MIGRATE_SYNC
This could break it.

Best regards,
Shivank