Re: [PATCH 1/3] mm/page_io: let block drivers register custom swap I/O ops

[Jianyue Wu]

On 6/15/2026 9:50 AM, YoungJun Park wrote:
> On Sun, Jun 14, 2026 at 11:35:29PM +0800, Jianyue Wu wrote:
>
> ...
>
> Hello Jianyue.
>
> Currently, the patch commit log indicates only a single custom swap
> registration is supported. Shouldn't we allow multiple block drivers to
> register their custom ops simultaneously from the beginning?
>
>> int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio,
>> struct list_head *folio_list);
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 284eebc40a70..ebdc96092961 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -2849,6 +2849,10 @@ static int setup_swap_extents(struct swap_info_struct *sis,
>> sis->ops = &swap_bdev_ops;
>>
>> if (S_ISBLK(inode->i_mode)) {
>> + const struct swap_ops *block_ops = lookup_swap_block_ops(sis);
>
> Also, just a personal thought on this part.
>
> Instead of using `block_device_fops` as a lookup key, what if we handle
> this similarly to how filesystems use the `a_ops->swap_activate` callback?
>
> We could add a `swap_activate` callback directly into
> struct block_device_operations (zram's zram_devops). This way, the
> block device itself can set up and replace the swap `ops` directly without
> needing a separate registration/lookup mechanism.
>
> What are your thoughts on this approach?
>
> Thanks,
> Youngjun Park
>

Hello Youngjun,

On multiple registrations:
Previously I was also a bit hesitate about this. Exactly, better to
support multiple block driver directly, I'll update it.

On swap_activate:
That's a very good idea, to use swap_activate callback, it is much
cleaner, I like this approach:) setup_swap_extents() would call it for
S_ISBLK swap targets, and the driver would install sis->ops at swapon
time. When the callback is NULL, the core can fall back to
swap_bdev_activate() and swap_bdev_ops. That removes the separate global
registration/lookup mechanism entirely, and multiple block drivers are
supported naturally because each device carries its own ops table.

Thanks,
Jianyue