linux-kernel - Re: [RFC PATCH 1/3] mm, memory_hotplug: try to migrate full section worth of pages

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Date:   Tue, 20 Nov 2018 14:33:53 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <OSalvador@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        David Hildenbrand <david@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC PATCH 1/3] mm, memory_hotplug: try to migrate full section
 worth of pages

On 18-11-20 14:43:21, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> do_migrate_range has been limiting the number of pages to migrate to 256
> for some reason which is not documented. Even if the limit made some
> sense back then when it was introduced it doesn't really serve a good
> purpose these days. If the range contains huge pages then
> we break out of the loop too early and go through LRU and pcp
> caches draining and scan_movable_pages is quite suboptimal.
> 
> The only reason to limit the number of pages I can think of is to reduce
> the potential time to react on the fatal signal. But even then the
> number of pages is a questionable metric because even a single page
> might migration block in a non-killable state (e.g. __unmap_and_move).
> 
> Remove the limit and offline the full requested range (this is one
> membblock worth of pages with the current code). Should we ever get a
> report that offlining takes too long to react on fatal signal then we
> should rather fix the core migration to use killable waits and bailout
> on a signal.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Looks good to me, I also do not see a reason for 256 pages limit.

Reviewed-by: Pavel Tatashin <pasha.tatashin@...een.com>

Added Kame to CC, who introduced page offlining, and this limit, but as
far as I can tell the last time he was active on LKML was in 2016.

Pasha