linux-kernel - Re: [RESEND PATCH] mm/hotplug: don't reset pagetype flags for offline

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Date:   Tue, 12 Mar 2019 16:57:30 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org, vbabka@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] mm/hotplug: don't reset pagetype flags for offline

On Tue 12-03-19 16:35:06, Oscar Salvador wrote:
[...]
> One thing I noticed when looking at start_isolate_page_range and 
> undo_isolate_page_range functions, is that start_isolate_page_range increments
> the number of isolated pageblocks, while undo_isolate_page_range does the counter
> part.
> Since undo_isolate_page_range is really never called during offlining,
> we leave zone->nr_isolate_pageblock with a stale value.
> 
> I __think__  this does not matter much.
> We only get to check whether a zone got isolated pageblocks in
> has_isolate_pageblock(), and this is called from:
> 
> free_one_page
> free_pcppages_bulk
> __free_one_page

It forces those into slow(er) path. So it makes a difference.

> With a quick glance, the only difference in has_isolate_pageblock() returning
> true or false, seems to be that those functions perform some extra checks in
> case the zone reports to have isolated pageblocks.
> 
> I wonder if we should set nr_isolate_pageblock back to its original value
> before start_isolate_page_range.

Yes. And that would be a fixup to my 2ce13640b3f4.

-- 
Michal Hocko
SUSE Labs