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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Joshua Hahn <joshua.hahnjy@gmail.com> writes:

> 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.

Because it's not an ABI, I think that we could avoid to say "never".

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

Welcome!  You too!

With some trivial tweak, please feel free to add my

Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>

in the future version.

---
Best Regards,
Huang, Ying