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

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Message-ID: <6020005e-8b62-415f-993e-b1d99e0c5158@kernel.org>
Date: Mon, 17 Nov 2025 13:58:19 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Balbir Singh <balbirs@...dia.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, dri-devel@...ts.freedesktop.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [PATCH] fixup: mm/rmap: extend rmap and migration support
 device-private entries

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@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
> Cc: Rakie Kim <rakie.kim@...com>
> Cc: Byungchul Park <byungchul@...com>
> Cc: Gregory Price <gourry@...rry.net>
> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Matthew Brost <matthew.brost@...el.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
> 
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
> 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