Re: [PATCH RFC v6 3/5] mm/migrate: add copy offload registration infrastructure

[Zi Yan]

On 20 Jul 2026, at 7:19, Huang, Ying wrote:

> Shivank Garg <shivankg@xxxxxxx> writes:
>
> [snip]
>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 41b732e78a67..4fed3110ca0a 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -43,6 +43,7 @@
>>  #include <linux/sched/sysctl.h>
>>  #include <linux/memory-tiers.h>
>>  #include <linux/pagewalk.h>
>> +#include <linux/migrate_copy_offload.h>
>>
>>  #include <asm/tlbflush.h>
>>
>> @@ -51,12 +52,6 @@
>>  #include "internal.h"
>>  #include "swap.h"
>>
>> -/* For now, never offload. Wired up in later patch. */
>> -static bool migrate_should_offload(int reason)
>> -{
>> -	return false;
>> -}
>> -
>>  static const struct movable_operations *offline_movable_ops;
>>  static const struct movable_operations *zsmalloc_movable_ops;
>>
>> @@ -2050,7 +2045,7 @@ static int migrate_pages_batch(struct list_head *from,
>>
>>  	/* Batch-copy eligible folios before the move phase */
>>  	if (!list_empty(&unmap_batch))
>> -		migrate_folios_mc_copy(&dst_batch, &unmap_batch, nr_batch);
>> +		migrate_offload_batch_copy(&dst_batch, &unmap_batch, nr_batch);
>>
>>  	retry = 1;
>>  	for (pass = 0; pass < nr_pass && retry; pass++) {
>
> Found a difference between v5 and v6 here.  In v5, migrate_pages_batch()
> checks the return value of migrate_offload_batch_copy() and acts
> accordingly. While in v6, it does not.  Why was this changed?
>
> IIUC, FOLIO_CONTENT_COPIED is now used to indicate a copy failure.  Is
> this necessary?  Is it possible that some (but not all) folios fail to
> be copied?

Based on the discussion[1], FOLIO_CONTENT_COPIED is used to indicate both
1. folio is copied via offloading, and 2. MC copy failure. So the code
no longer needs to store the return value of migrate_offload_batch_copy()
and can just check dst->migrate_info & FOLIO_CONTENT_COPIED.

For 32bit, FOLIO_CONTENT_COPIED is always 0, so all folios are always
copied in the serialized path. Hmm, that might be causing double copying,
since migrate_folios_mc_copy() copies batched folios and later
migrate_folios_move() copies them again since
dst->migrate_info & FOLIO_CONTENT_COPIED is always false. Maybe we could
make migrate_folios_mc_copy() return immediately in 32bit, otherwise,
we will need to find somewhere else to store FOLIO_CONTENT_COPIED.

[1] https://lore.kernel.org/all/c2bfdf39-8caa-43bd-b1ad-2285b4cc767a@xxxxxxx/

>
> [snip]
>
> ---
> Best Regards,
> Huang, Ying


Best Regards,
Yan, Zi