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

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

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

> On Mon, 04 Aug 2025 09:24:31 +0800 "Huang, Ying" <ying.huang@linux.alibaba.com> wrote:
>
>> 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.
>
> Hi Ying,
>
>> Because it's not an ABI, I think that we could avoid to say "never".
>
> My personal opinion is that we should keep this warning, since there has
> already been an example before where a developer tried to remove this bit [1],
> and this broke some behavior for userspace configurations. However, if I
> understand your comment correctly, you are suggesting that we should change
> the wording to not include "never", since sysctls are no longer an ABI (and
> therefore we should be OK to change what the values mean?)
>
> If that is the case, then I can send in another patch since I think the goals
> are a bit different for the two patches. With that said, I think we should
> keep the warning just to avoid any breakages in userspace, even if sysctl
> might not be considered an ABI anymore (also I must have missed this, I didn't
> know this at all!)

Sorry for confusing.  I agree that we shouldn't change the sysctl
interface in most cases.  I just thought that we could soften the
wording a little?  For example,

New bits are OK, but existing bits shouldn't be changed.

I think that it's still clear that we don't want to change the existing
bits.

However, my English is poor.  So, my suggestion may not make sense.

>> > 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.
>
> Thank you for your review Ying! Since there is a question remaining about what
> to do with the "never" statement, I will wait to send out a v3 with your
> review : -) 

---
Best Regards,
Huang, Ying