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

Garg, Shivank
2026-04-03T11:11:28Z

On 3/24/2026 4:24 PM, Huang, Ying wrote:
> Shivank Garg <shivankg@amd.com> 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