Re: [PATCH 2/3] mm/mempolicy: refactor a read-once mechanism into a function for re-use

Huang, Ying
2024-01-15T04:15:09Z

Gregory Price <gourry.memverge@gmail.com> writes:

> move the use of barrier() to force policy->nodemask onto the stack into
> a function `read_once_policy_nodemask` so that it may be re-used.
>
> Suggested-by: Huang Ying <ying.huang@intel.com>
> Signed-off-by: Gregory Price <gregory.price@memverge.com>
> ---
> mm/mempolicy.c | 26 ++++++++++++++++----------
> 1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 5da4fd79fd18..0abd3a3394ef 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1907,6 +1907,20 @@ unsigned int mempolicy_slab_node(void)
> }
> }
>
> +static unsigned int read_once_policy_nodemask(struct mempolicy *pol,
> + nodemask_t *mask)

It may be more useful if we define this as memcpy_once(). That can be
used not only for nodemask, but also other data structure.

> +{
> + /*
> + * barrier stabilizes the nodemask locally so that it can be iterated
> + * over safely without concern for changes. Allocators validate node
> + * selection does not violate mems_allowed, so this is safe.
> + */
> + barrier();
> + __builtin_memcpy(mask, &pol->nodes, sizeof(nodemask_t));

We don't use __builtin_memcpy() in kernel itself directly. Although it
is used in kernel tools. So, I think it's better to use memcpy() here.

> + barrier();
> + return nodes_weight(*mask);
> +}
> +
> /*
> * Do static interleaving for interleave index @ilx. Returns the ilx'th
> * node in pol->nodes (starting from ilx=0), wrapping around if ilx
> @@ -1914,20 +1928,12 @@ unsigned int mempolicy_slab_node(void)
> */
> static unsigned int interleave_nid(struct mempolicy *pol, pgoff_t ilx)
> {
> - nodemask_t nodemask = pol->nodes;
> + nodemask_t nodemask;
> unsigned int target, nnodes;
> int i;
> int nid;
> - /*
> - * The barrier will stabilize the nodemask in a register or on
> - * the stack so that it will stop changing under the code.
> - *
> - * Between first_node() and next_node(), pol->nodes could be changed
> - * by other threads. So we put pol->nodes in a local stack.
> - */
> - barrier();
>
> - nnodes = nodes_weight(nodemask);
> + nnodes = read_once_policy_nodemask(pol, &nodemask);
> if (!nnodes)
> return numa_node_id();
> target = ilx % nnodes;

--
Best Regards,
Huang, Ying