linux-kernel - Re: [v5 03/15] mm/rmap: extend rmap and migration support device-private entries

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Message-ID: <6f1857b8-45db-4017-b6e8-02ecfa2104a3@redhat.com>
Date: Thu, 11 Sep 2025 14:04:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
 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: [v5 03/15] mm/rmap: extend rmap and migration support
 device-private entries

On 08.09.25 02:04, Balbir Singh wrote:

subject:

"mm/rmap: rmap and migration support for device-private PMD entries"

> Add device-private THP support to reverse mapping infrastructure,
> enabling proper handling during migration and walk operations.
> 
> The key changes are:
> - add_migration_pmd()/remove_migration_pmd(): Handle device-private
>    entries during folio migration and splitting
> - page_vma_mapped_walk(): Recognize device-private THP entries during
>    VMA traversal operations
> 
> This change supports folio splitting and migration operations on
> device-private entries.
> 
> 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>
> ---

[...]

> +++ b/mm/page_vma_mapped.c
> @@ -250,12 +250,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>   			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>   			pmde = *pvmw->pmd;
>   			if (!pmd_present(pmde)) {
> -				swp_entry_t entry;
> +				swp_entry_t entry = pmd_to_swp_entry(pmde);
>   
>   				if (!thp_migration_supported() ||
>   				    !(pvmw->flags & PVMW_MIGRATION))
>   					return not_found(pvmw);
> -				entry = pmd_to_swp_entry(pmde);
>   				if (!is_migration_entry(entry) ||
>   				    !check_pmd(swp_offset_pfn(entry), pvmw))
>   					return not_found(pvmw);

Why this change? Looks unrelated.

> @@ -277,6 +276,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>   			 * cannot return prematurely, while zap_huge_pmd() has
>   			 * cleared *pmd but not decremented compound_mapcount().

Reminder to self: cleanup compound_mapcount() leftovers

>   			 */
> +			swp_entry_t entry;
> +
> +			entry = pmd_to_swp_entry(pmde);

swp_entry_t entry = pmd_to_swp_entry(pmde);

> +
> +			if (is_device_private_entry(entry)) {
> +				pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> +				return true;
> +			}
> +
>   			if ((pvmw->flags & PVMW_SYNC) &&
>   			    thp_vma_suitable_order(vma, pvmw->address,
>   						   PMD_ORDER) &&
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 236ceff5b276..6de1baf7a4f1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1063,8 +1063,10 @@ static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
>   		} else {
>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>   			pmd_t *pmd = pvmw->pmd;
> -			pmd_t entry;
> +			pmd_t entry = pmdp_get(pmd);
>   
> +			if (!pmd_present(entry))
> +				continue;
>   			if (!pmd_dirty(*pmd) && !pmd_write(*pmd))
>   				continue;
>   

If you just did a pmdp_get() you should use it in these functions as 
well. If not (cleanup later), do a straight *pmd like the others.

Apart from that nothing jumped at me.

-- 
Cheers

David / dhildenb