Re: [PATCH] mm, memory_hotplug: check zone_movable in has_unmovable_pages

[Balbir Singh <bsingharora@gmail.com>]

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@gmail.com>

Balbir Singh.