Re: [PATCH v3] mempolicy: Clarify what zone reclaim means

[Byungchul Park <byungchul@sk.com>]

On Tue, Aug 05, 2025 at 01:50:47PM -0700, Joshua Hahn wrote:
> The zone_reclaim_mode API controls the reclaim behavior when a node runs out of
> memory. Contrary to its user-facing name, it is internally referred to as
> "node_reclaim_mode".
> 
> This can be confusing. But because we cannot change the name of the API since
> it has been in place since at least 2.6, let's try to be more explicit about
> what the behavior of this API is.
> 
> Change the description to clarify what zone reclaim entails, and be explicit
> about the RECLAIM_ZONE bit, whose purpose has led to some confusion in the
> past already [1] [2].
> 
> While at it, also soften the warning about changing these bits.
> 
> [1] https://lore.kernel.org/linux-mm/1579005573-58923-1-git-send-email-alex.shi@linux.alibaba.com/
> [2] https://lore.kernel.org/linux-mm/20200626003459.D8E015CA@viggo.jf.intel.com/
> 
> Acked-by: SeongJae Park <sj@kernel.org>
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> ---
> v2 --> v3:
> - Fixed typos
> - Softend wording from "never" --> "should not"
> 
>  include/uapi/linux/mempolicy.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> index 1f9bb10d1a47..683c130782f0 100644
> --- a/include/uapi/linux/mempolicy.h
> +++ b/include/uapi/linux/mempolicy.h
> @@ -66,10 +66,16 @@ enum {
>  #define MPOL_F_MORON   (1 << 4) /* Migrate On protnone Reference On Node */
> 
>  /*
> + * Enabling zone reclaim means the page allocator will attempt to fulfill
> + * the allocation request on the current node by triggering reclaim and
> + * trying to shrink the current node.
> + * Fallback allocations on the next candidates in the zonelist are considered
> + * when reclaim fails to free up enough memory in the current node/zone.

I was confused too, at the beginning.  Thanks for the explicit comment.

Acked-by: Byungchul Park <byungchul@sk.com>

	Byungchul

> + *
>   * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
> - * ABI.  New bits are OK, but existing bits can never change.
> + * ABI.  New bits are OK, but existing bits should not be changed.
>   */
> -#define RECLAIM_ZONE   (1<<0)  /* Run shrink_inactive_list on the zone */
> +#define RECLAIM_ZONE   (1<<0)  /* Enable zone reclaim */
>  #define RECLAIM_WRITE  (1<<1)  /* Writeout pages during reclaim */
>  #define RECLAIM_UNMAP  (1<<2)  /* Unmap pages during reclaim */
> 
> 
> base-commit: 6bcdbd62bd56e6d7383f9e06d9d148935b3c9b73
> --
> 2.47.3