Re: [RFC PATCH v4 4/6] mm/migrate: add copy offload registration infrastructure

[Garg, Shivank]

On 3/24/2026 4:24 PM, Huang, Ying wrote:
> Shivank Garg <shivankg@xxxxxxx> writes:
> 
>> Introduce CONFIG_MIGRATION_COPY_OFFLOAD, which lets offload driver
> 
> Do we really need a new kconfig option?  IMHO, we have too many now.
> Because we have a jump label already, the performance difference should
> be trivial.  Can you measure the size difference?

BASELINE (offload=n)
   text    data     bss     dec  filename
  23577    1632      32   25241  mm/migrate.o
39202900        14159750        6502152 59864802     vmlinux

WITH OFFLOAD (offload=y)
   text    data     bss     dec  filename
  24444    2568      32   27044  mm/migrate.o
    676      64       8     748  mm/migrate_copy_offload.o
39208218        14163942        6498120 59870280     vmlinux

WITHOUT CONFIG (always-on)
   text    data     bss     dec  filename
  24444    2568      32   27044  mm/migrate.o
    676      64       8     748  mm/migrate_copy_offload.o
39208405        14163942        6498120 59870467     vmlinux

It saves around 5.5KB of size, when offload support is disabled.
Is it meaningful savings? What do you think?

> 
>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
>> +extern struct static_key_false migrate_offload_enabled;
>> +extern struct srcu_struct migrate_offload_srcu;
>> +bool migrate_should_batch_default(int reason);
>> +int migrate_offload_start(struct migrator *m);
>> +int migrate_offload_stop(struct migrator *m);
> 
> Why not naming the function migrate_offload_register/unregister()?
> IMHO, that sounds more natural.

Ack. I'll rename to migrate_offload_register/unregister().

> 
>>  
>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
>> +	/* Check if the offload driver wants to batch for this reason */
>> +	if (static_branch_unlikely(&migrate_offload_enabled))
>> +		do_batch = static_call(migrate_should_batch)(reason);
> 
> Should batching based on "reason" be determined by the general migrate
> code instead of the migrator implementation?  For example, if we only
> batch copying for ASYNC migration, we should determine that in
> migrate_pages_batch() instead of the migreation implementation.  Or am I
> missed something?  If so, can you provide an example?
> 

My idea was that different drivers may have different cost/benefit
profiles(e.g. setup cost, migrate batch-size, etc..) 

For instance, a DMA driver may want to target only bulk migration usecase.
And a CPU-thread based driver can be used more broadly, without worrying
about setup-costs.

But I agree it's premature with only one-driver.
I'll move the reason check with target usecases into migrate_pages_batch()
and drop the should_batch() callback. If a future driver needs different
filtering, we can add it back then.

>>  
>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
>>  	/* Batch-copy eligible folios before the move phase */
>>  	if (!list_empty(&src_batch)) {
> 
> Guard with "static_branch_unlikely(&migrate_offload_enabled)" first?
> Better to define a inline function to shorten the expression.
> 

Sure, will add the static_branch_unlikely guard and wrap in a helper
function. Thanks.

Best regards,
Shivank