linux-kernel - Re: [PATCH] mm/huge_memory: fix override of entry in remove_migration

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Message-ID: <e8f4546a-5c48-4488-a115-5af65c06f1dc@kernel.org>
Date: Fri, 14 Nov 2025 10:23:02 +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] mm/huge_memory: fix override of entry in
 remove_migration_pmd

On 14.11.25 02:21, Balbir Singh wrote:
> Recent changes exposed a BUG in remove_migration_pmd() where the
> migration entry was being overridden when the folio is device private.
> 
> Use scope local entry for creating the device private pmde. Make the
> pmde writable if the migration entry is writable by moving the check
> is_migration_write() prior to creating the device private entry.
> 
> 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>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>   mm/huge_memory.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9dda8c48daca..df93768a6e15 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4698,7 +4698,12 @@ 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 (is_writable_migration_entry(entry))
> +		pmde = pmd_mkwrite(pmde, vma);
> +
>   	if (folio_is_device_private(folio)) {
> +		swp_entry_t entry;
> +
>   		if (pmd_write(pmde))
>   			entry = make_writable_device_private_entry(
>   							page_to_pfn(new));
> @@ -4710,8 +4715,6 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>   
>   	if (pmd_swp_soft_dirty(*pvmw->pmd))
>   		pmde = pmd_mksoft_dirty(pmde);
> -	if (is_writable_migration_entry(entry))
> -		pmde = pmd_mkwrite(pmde, vma);
>   	if (pmd_swp_uffd_wp(*pvmw->pmd))
>   		pmde = pmd_mkuffd_wp(pmde);
>   	if (!is_migration_entry_young(entry))

There are more problems here: you cannot call pmd_mksoft_dirty() etc on 
something that is not a present pmd! We have pmd_swp_mksoft_dirty() and 
friends for that.

So you'll have to completely split both paths.

-- 
Cheers

David