linux-kernel - Re: [PATCH] mm/vmscan: restore zone_reclaim

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Message-ID: <83731eeb-1f64-50b7-41e9-5b7114678533@intel.com>
Date:   Fri, 26 Jun 2020 06:53:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Daniel Wagner <dwagner@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        ben.widawsky@...el.com, alex.shi@...ux.alibaba.com,
        tobin@...nel.org, cl@...ux.com, akpm@...ux-foundation.org,
        stable@...nel.org
Subject: Re: [PATCH] mm/vmscan: restore zone_reclaim_mode ABI

On 6/26/20 12:59 AM, Daniel Wagner wrote:
>> I went to go add a new RECLAIM_* mode for the zone_reclaim_mode
>> sysctl.  Like a good kernel developer, I also went to go update the
>> documentation.  I noticed that the bits in the documentation didn't
>> match the bits in the #defines.
> Drop the this paragraph from the commit message. It doesn't add
> any necessart information.
> 
> Please have a look at
> 
> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

Sure!  The first paragraph says:

	Describe your problem. ... there must be an underlying
	problem that motivated you to do this work.

This describes what motivated my work and whether it caused an actual
versus theoretical underlying problem.

Reviewers and maintainers often want to know the impact of a bug fix.
It's important to convey whether this was found because it caused
datacenters in 14 states to catch fire or whether it was found by
inspection.

This provides the information that it was found by inspection.

Was there something else specifically in the documentation which you
think I've neglected?

>> -zone_reclaim may be enabled if it's known that the workload is partitioned
>> -such that each partition fits within a NUMA node and that accessing remote
>> -memory would cause a measurable performance reduction.  The page allocator
>> -will then reclaim easily reusable pages (those page cache pages that are
>> -currently not used) before allocating off node pages.
>> +Consider enabling one or more zone_reclaim mode bits if it's known that the
>> +workload is partitioned such that each partition fits within a NUMA node
>> +and that accessing remote memory would cause a measurable performance
>> +reduction.  The page allocator will take additional actions before
>> +allocating off node pages.
> 
> I think the documentation update should not be part of this patch.
> This makes the back porting to stable more difficult.

Really?  If a backporter doesn't care about documentation, I'd just
expect them to see the reject, ignore it, and move on with their life.
If they do, they'd want the code fix and the Documentation/ update in
the same patch so that they don't get disconnected.