Re: [PATCH v2] mempolicy: Clarify what zone reclaim means

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

On Fri, 01 Aug 2025 08:59:20 +0800 "Huang, Ying" <ying.huang@linux.alibaba.com> wrote:

> Joshua Hahn <joshua.hahnjy@gmail.com> writes:
> 
> > 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].
> >
> > [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/
> >
> > Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> > ---
> >  include/uapi/linux/mempolicy.h | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> > index 1f9bb10d1a47..6c9c9385ff89 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
> > + * zone when reclaim fails to free up enough memory in the current node/zone.
> > + *
> >   * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
> >   * ABI.  New bits are OK, but existing bits can never change.
> 
> As far as I know, sysctl isn't considered kernel ABI now.  So, cghane
> this line too?

Hi Ying, 

Thank you for reviewing this patch!

I didn't know that sysctl isn't considered a kernel ABI. If I understand your
suggestion correctly, I can rephrase the comment block above to something like this?

- * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
- * ABI. New bits are OK, but existing bits can never change.
+ * These bit locations are exposed in the vm.zone_reclaim_mode sysctl and
+ * in /proc/sys/vm/zone_reclaim_mode. New bits are OK, but existing bits
+ * can never change.

Thanks again for your review Ying, I hope you have a good day : -)
Joshua

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