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Subject: Re: [PATCH] mm/vmscan: restore zone_reclaim_mode ABI
To:     Baoquan He <bhe@redhat.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        ben.widawsky@intel.com, alex.shi@linux.alibaba.com,
        dwagner@suse.de, tobin@kernel.org, cl@linux.com,
        akpm@linux-foundation.org, stable@kernel.org
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <791c47ad-5a6b-1f1b-c34b-d8bbf7722957@intel.com>
Date:   Mon, 29 Jun 2020 16:37:37 -0700
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On 6/29/20 4:30 PM, Baoquan He wrote:
>> The only way I can plausibly think of "cleaning up" the RECLAIM_ZONE bit
>> would be to raise our confidence that it is truly unused.  That takes
>> time, and probably a warning if we see it being set.  If we don't run
>> into anybody setting it or depending on it being set in a few years, we
>> can remove it.
> So adding the old bit back for compatibility looks good, thanks.
> 
> Then we have to be very careful when adding and reviewing new
> interface introducing, should not leave one which might be used
> in the future.
> 
> In fact, RECLAIM_ZONE is not completely useless. At least, when the old
> bit 0 is set, it may enter into node_reclaim() in get_page_from_freelist(),
> that makes it like a switch.
> 
> get_page_from_freelist {
> 
> 	...
>                         if (node_reclaim_mode == 0 ||                                                                                             
>                             !zone_allows_reclaim(ac->preferred_zoneref->zone, zone))
>                                 continue;
> 	...
> }

Oh, that's a very good point.  There are a couple of those around.  Let
me circle back and update the documentation and the variable name.  I'll
send out another version.