Re: [PATCH 1/2] mm/mempolicy: merge adjacent shared policy ranges

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Fri, Nov 13, 2026 at 10:36:12PM +0100, David Hildenbrand (Arm) wrote:
> Shared policies can retain separate tree nodes for adjacent ranges with
> identical policies. This can create unnecessary policy ranges especially
> when individual VMAs cover individual file ranges, and user space sets
> the policy on these VMAs independently.
> 
> Extend the existing replacement handling to merge ranges with equal policy
> at either boundary. The existing replacement loop will then remove the old
> nodes and installs a single node for the combined range. This also avoids
> splitting an existing node when setting the same policy on a subrange.
> 
> Extending either boundary rules out splitting an existing node, so the
> allocation retry can only occur with the originally requested range.

Hi David,

Independent of 2/2 I think this is a really great change. Thank you!
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>

> This change will make it easier to cap the number of shared policy
> ranges per inode.
> 
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
>  mm/mempolicy.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 06b90903e8794..183bc3bd077cd 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -3208,6 +3208,23 @@ static int shared_policy_replace(struct shared_policy *sp, pgoff_t start,
>  
>  restart:
>  	write_lock(&sp->lock);
> +	if (new) {
> +		/* Try to merge ranges with same policies. */
> +		if (start) {
> +			n = sp_lookup(sp, start - 1, start);
> +			if (n && mpol_equal(n->policy, new->policy)) {
> +				new->start = n->start;
> +				start = new->start;
> +			}
> +		}
> +		if (end != ULONG_MAX) {
> +			n = sp_lookup(sp, end, end + 1);
> +			if (n && mpol_equal(n->policy, new->policy)) {
> +				new->end = n->end;
> +				end = new->end;
> +			}
> +		}
> +	}
>  	n = sp_lookup(sp, start, end);
>  	/* Take care of old policies in the same range. */
>  	while (n && n->start < end) {
> 
> -- 
> 2.43.0
>