linux-kernel - Re: [PATCH] mm, memory_hotplug: check zone_movable in has_unmovable

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Date:   Fri, 9 Nov 2018 21:45:41 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Oscar Salvador <OSalvador@...e.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, memory_hotplug: check zone_movable in
 has_unmovable_pages

On Wed, Nov 07, 2018 at 02:06:55PM +0100, Michal Hocko wrote:
> On Wed 07-11-18 23:53:24, Balbir Singh wrote:
> > On Wed, Nov 07, 2018 at 08:35:48AM +0100, Michal Hocko wrote:
> > > On Wed 07-11-18 07:35:18, Balbir Singh wrote:
> [...]
> > > > The check seems to be quite aggressive and in a loop that iterates
> > > > pages, but has nothing to do with the page, did you mean to make
> > > > the check
> > > > 
> > > > zone_idx(page_zone(page)) == ZONE_MOVABLE
> > > 
> > > Does it make any difference? Can we actually encounter a page from a
> > > different zone here?
> > > 
> > 
> > Just to avoid page state related issues, do we want to go ahead
> > with the migration if zone_idx(page_zone(page)) != ZONE_MOVABLE.
> 
> Could you be more specific what kind of state related issues you have in
> mind?
> 

I was wondering if page_zone() is setup correctly, but it's setup
upfront, so I don't think that is ever an issue.

> > > > it also skips all checks for pinned pages and other checks
> > > 
> > > Yes, this is intentional and the comment tries to explain why. I wish we
> > > could be add a more specific checks for movable pages - e.g. detect long
> > > term pins that would prevent migration - but we do not have any facility
> > > for that. Please note that the worst case of a false positive is a
> > > repeated migration failure and user has a way to break out of migration
> > > by a signal.
> > >
> > 
> > Basically isolate_pages() will fail as opposed to hotplug failing upfront.
> > The basic assertion this patch makes is that all ZONE_MOVABLE pages that
> > are not reserved are hotpluggable.
> 
> Yes, that is correct.
>

I wonder if it is easier to catch a __SetPageReserved() on ZONE_MOVABLE memory
at set time, the downside is that we never know if that memory will ever be
hot(un)plugged. The patch itself, I think is OK

Acked-by: Balbir Singh <bsingharora@...il.com>

Balbir Singh.