Re: [PATCH] mm/huge_memory: fix override of entry in remove_migration_pmd

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Nov 13, 2025 at 04:13:52PM +1100, Balbir Singh wrote:
> The softleaf changes exposed a BUG in remove_rmap_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
> softleaf_is_migration_write() prior to creating the device private entry.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> Cc: Rakie Kim <rakie.kim@sk.com>
> Cc: Byungchul Park <byungchul@sk.com>
> Cc: Gregory Price <gourry@gourry.net>
> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Mika Penttilä <mpenttil@redhat.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>

THe logic LGTM but we don't want to have a bisect hazard here by having the bug
introduced earlier then resolved here.

I think better would be to do the fix up in the original series as a fix-patch,
then maybe point at this one as a conflict resolution for my series?

In any case, LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.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 0184cd915f44..e4dadcb9e90b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4850,7 +4850,12 @@ 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 (softleaf_is_migration_write(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));
> @@ -4862,8 +4867,6 @@ 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 (softleaf_is_migration_write(entry))
> -		pmde = pmd_mkwrite(pmde, vma);
>  	if (pmd_swp_uffd_wp(*pvmw->pmd))
>  		pmde = pmd_mkuffd_wp(pmde);
>  	if (!softleaf_is_migration_young(entry))
> --
> 2.51.1
>