Re: [PATCH 6.6.y] mm/mempolicy: fix wrong mmap_read_unlock() in migrate_to_node()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* David Hildenbrand (Arm) <david@kernel.org> [260303 05:12]:
> The backport of commit 091c1dd2d4df ("mm/mempolicy: fix migrate_to_node()
> assuming there is at least one VMA in a MM") contains an error:
> migrate_to_node() does not lock the mmap_lock itself, that is handled by
> the caller instead.
> 
> So let's drop the wrong mmap_read_unlock(). Fortunately, this path is
> very hard to hit in practice.
> 
> Fixes: a13b2b9b0b0b ("mm/mempolicy: fix migrate_to_node() assuming there is at least one VMA in a MM")
> Reported-by: Li Ying <liying3@sungrowpower.com>
> Closes: https://lore.kernel.org/r/aaZgUNxAyKC2IwuG@casper.infradead.org
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Ouch, yeah.  The locking is different.

Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>

> ---
>  mm/mempolicy.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 94c74c594d10..d8007e1c2690 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1071,10 +1071,8 @@ static long migrate_to_node(struct mm_struct *mm, int source, int dest,
>  
>  	VM_BUG_ON(!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)));
>  	vma = find_vma(mm, 0);
> -	if (unlikely(!vma)) {
> -		mmap_read_unlock(mm);
> +	if (unlikely(!vma))
>  		return 0;
> -	}
>  
>  	/*
>  	 * This does not migrate the range, but isolates all pages that
> -- 
> 2.43.0
>