Re: [PATCH v1 1/2] mm/userfaultfd: fix uffd-wp handling for THP migration entries

[Peter Xu <peterx@redhat.com>]

On Wed, Apr 05, 2023 at 04:25:34PM +0200, David Hildenbrand wrote:
> Looks like what we fixed for hugetlb in commit 44f86392bdd1 ("mm/hugetlb:
> fix uffd-wp handling for migration entries in hugetlb_change_protection()")
> similarly applies to THP.
> 
> Setting/clearing uffd-wp on THP migration entries is not implemented
> properly. Further, while removing migration PMDs considers the uffd-wp
> bit, inserting migration PMDs does not consider the uffd-wp bit.
> 
> We have to set/clear independently of the migration entry type in
> change_huge_pmd() and properly copy the uffd-wp bit in
> set_pmd_migration_entry().
> 
> Verified using a simple reproducer that triggers migration of a THP, that
> the set_pmd_migration_entry() no longer loses the uffd-wp bit.
> 
> Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks, one trivial nitpick:

> ---
>  mm/huge_memory.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 032fb0ef9cd1..bdda4f426d58 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1838,10 +1838,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  	if (is_swap_pmd(*pmd)) {
>  		swp_entry_t entry = pmd_to_swp_entry(*pmd);
>  		struct page *page = pfn_swap_entry_to_page(entry);
> +		pmd_t newpmd;
>  
>  		VM_BUG_ON(!is_pmd_migration_entry(*pmd));
>  		if (is_writable_migration_entry(entry)) {
> -			pmd_t newpmd;
>  			/*
>  			 * A protection check is difficult so
>  			 * just be safe and disable write
> @@ -1855,8 +1855,16 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  				newpmd = pmd_swp_mksoft_dirty(newpmd);
>  			if (pmd_swp_uffd_wp(*pmd))
>  				newpmd = pmd_swp_mkuffd_wp(newpmd);
> -			set_pmd_at(mm, addr, pmd, newpmd);
> +		} else {
> +			newpmd = *pmd;
>  		}
> +
> +		if (uffd_wp)
> +			newpmd = pmd_swp_mkuffd_wp(newpmd);
> +		else if (uffd_wp_resolve)
> +			newpmd = pmd_swp_clear_uffd_wp(newpmd);
> +		if (!pmd_same(*pmd, newpmd))
> +			set_pmd_at(mm, addr, pmd, newpmd);
>  		goto unlock;
>  	}
>  #endif
> @@ -3251,6 +3259,8 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
>  	pmdswp = swp_entry_to_pmd(entry);
>  	if (pmd_soft_dirty(pmdval))
>  		pmdswp = pmd_swp_mksoft_dirty(pmdswp);
> +	if (pmd_swp_uffd_wp(*pvmw->pmd))
> +		pmdswp = pmd_swp_mkuffd_wp(pmdswp);

I think it's fine to use *pmd, but maybe still better to use pmdval?  I
worry pmdp_invalidate()) can be something else in the future that may
affect the bit.

>  	set_pmd_at(mm, address, pvmw->pmd, pmdswp);
>  	page_remove_rmap(page, vma, true);
>  	put_page(page);
> -- 
> 2.39.2
> 

-- 
Peter Xu