Re: [PATCH 1/4] mm/migrate: Add migrate_device_split_page

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Dec 16, 2025 at 08:34:46PM +0000, Matthew Wilcox wrote:
> On Tue, Dec 16, 2025 at 09:10:11PM +0100, Francois Dugast wrote:
> > +	ret = __split_unmapped_folio(folio, 0, page, NULL, NULL, SPLIT_TYPE_UNIFORM);
> 
> We're trying to get rid of uniform splits.  Why do you need this to be
> uniform?

It’s very possible we’re doing this incorrectly due to a lack of core MM
experience. I believe Zi Yan suggested this approach (use
__split_unmapped_folio) a while back.

Let me start by explaining what we’re trying to do and see if there’s a
better suggestion for how to accomplish it.

Would SPLIT_TYPE_NON_UNIFORM split work here? Or do you have another
suggestion on how to split the folio aside from __split_unmapped_folio?

This covers the case where a GPU device page was allocated as a THP
(e.g., we call zone_device_folio_init with an order of 9). Later, this
page is freed/unmapped and then reallocated for a CPU VMA that is
smaller than a THP (e.g., we’d allocate either 4KB or 64KB based on
CPU VMA size alignment). At this point, we need to split the device
folio so we can migrate data into 4KB device pages.

Would SPLIT_TYPE_NON_UNIFORM work here? Or do you have another
suggestion for splitting the folio aside from __split_unmapped_folio?

Matt