Re: [PATCH 04/13] mm/huge_memory: move softleaf_to_folio() inside migration branch

[Dev Jain <dev.jain@arm.com>]

On 27/04/26 3:31 pm, Usama Arif wrote:
> change_non_present_huge_pmd() calls softleaf_to_folio() unconditionally
> at the top of the function.  softleaf_to_folio() extracts a PFN from
> the entry and converts it to a folio pointer, which is only meaningful
> for migration and device_private entries that encode a real PFN.
> 
> A swap entry encodes a swap offset instead, so softleaf_to_folio()
> would produce a bogus pointer and crash on mprotect() when a PMD swap
> entry is present.
> 
> Move the call into the migration_write branch where the folio is
> actually used, so the function is safe for any non-present PMD type.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---

Reviewed-by: Dev Jain <dev.jain@arm.com>

>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 49da0746b8ca..d82a19b5e276 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2619,11 +2619,12 @@ static void change_non_present_huge_pmd(struct mm_struct *mm,
>  		bool uffd_wp_resolve)
>  {
>  	softleaf_t entry = softleaf_from_pmd(*pmd);
> -	const struct folio *folio = softleaf_to_folio(entry);
>  	pmd_t newpmd;
>  
>  	VM_WARN_ON(!pmd_is_valid_softleaf(*pmd));
>  	if (softleaf_is_migration_write(entry)) {
> +		const struct folio *folio = softleaf_to_folio(entry);
> +
>  		/*
>  		 * A protection check is difficult so
>  		 * just be safe and disable write