Re: [PATCH] fixup: mm/rmap: extend rmap and migration support device-private entries

[David Hildenbrand (Red Hat)]

On 15.11.25 01:28, Balbir Singh wrote:
> Follow the pattern used in remove_migration_pte() in
> remove_migration_pmd(). Process the migration entries and if the entry
> type is device private, override the pmde with a device private entry
> and set the soft dirty and uffd_wp bits with the pmd_swp_mksoft_dirty
> and pmd_swp_mkuffd_wp
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> Cc: Rakie Kim <rakie.kim@xxxxxx>
> Cc: Byungchul Park <byungchul@xxxxxx>
> Cc: Gregory Price <gourry@xxxxxxxxxx>
> Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> Cc: Nico Pache <npache@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Dev Jain <dev.jain@xxxxxxx>
> Cc: Barry Song <baohua@xxxxxxxxxx>
> Cc: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Simona Vetter <simona@xxxxxxxx>
> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
>
> Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
> ---
> This fixup should be squashed into the patch "mm/rmap: extend rmap and
> migration support" of mm/mm-unstable
>
>   mm/huge_memory.c | 27 +++++++++++++++++----------
>   1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9dda8c48daca..50ba458efcab 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4698,16 +4698,6 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>   	folio_get(folio);
>   	pmde = folio_mk_pmd(folio, READ_ONCE(vma->vm_page_prot));
>   
> -	if (folio_is_device_private(folio)) {
> -		if (pmd_write(pmde))
> -			entry = make_writable_device_private_entry(
> -							page_to_pfn(new));
> -		else
> -			entry = make_readable_device_private_entry(
> -							page_to_pfn(new));
> -		pmde = swp_entry_to_pmd(entry);
> -	}
> -
>   	if (pmd_swp_soft_dirty(*pvmw->pmd))
>   		pmde = pmd_mksoft_dirty(pmde);
>   	if (is_writable_migration_entry(entry))
> @@ -4720,6 +4710,23 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>   	if (folio_test_dirty(folio) && is_migration_entry_dirty(entry))
>   		pmde = pmd_mkdirty(pmde);
>   
> +	if (folio_is_device_private(folio)) {
> +		swp_entry_t entry;

It's a bit nasty to have the same variable shadowed here.

We could reuse the existing entry by handling the code more similar to 
remove_migration_pte(): determine RMAP_EXCLUSIVE earlier.

> +
> +		if (pmd_write(pmde))
> +			entry = make_writable_device_private_entry(
> +							page_to_pfn(new));
> +		else
> +			entry = make_readable_device_private_entry(
> +							page_to_pfn(new));
> +		pmde = swp_entry_to_pmd(entry);
> +
> +		if (pmd_swp_soft_dirty(*pvmw->pmd))
> +			pmde = pmd_swp_mksoft_dirty(pmde);
> +		if (pmd_swp_uffd_wp(*pvmw->pmd))
> +			pmde = pmd_swp_mkuffd_wp(pmde);
> +	}
> +
>   	if (folio_test_anon(folio)) {
>   		rmap_t rmap_flags = RMAP_NONE;
>   

I guess at some point we could separate both parts completely (no need 
to do all this work on pmdb before the folio_is_device_private(folio) 
check, so this could be

if (folio_is_device_private(folio)) {
	...
} else {
	entry = pmd_to_swp_entry(*pvmw->pmd);
	folio_get(folio);
	...
}

That is something for another day though, and remove_migration_pte() 
should be cleaned up then as well.

--
Cheers

David