linux-kernel - Re: [PATCH v1 2/4] mm/memory_hotplug: Handle unaligned start and nr_pages in online_pages

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Message-ID: <20190814142834.GD17933@dhcp22.suse.cz>
Date:   Wed, 14 Aug 2019 16:28:34 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Arun KS <arunks@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v1 2/4] mm/memory_hotplug: Handle unaligned start and
 nr_pages in online_pages_blocks()

On Wed 14-08-19 16:17:03, David Hildenbrand wrote:
> On 14.08.19 16:08, Michal Hocko wrote:
> > On Fri 09-08-19 14:56:59, David Hildenbrand wrote:
> >> Take care of nr_pages not being a power of two and start not being
> >> properly aligned. Essentially, what walk_system_ram_range() could provide
> >> to us. get_order() will round-up in case it's not a power of two.
> >>
> >> This should only apply to memory blocks that contain strange memory
> >> resources (especially with holes), not to ordinary DIMMs.
> > 
> > I would really like to see an example of such setup before making the
> > code hard to read. Because I am not really sure something like that
> > exists at all.
> 
> I don't have a real-live example at hand (founds this while exploring
> the code), however, the linked commit changed it without stating why it
> would be safe to do so.

Then just drop the change. It is making the code a real head scratcher,
and more so after the next patch. I am pretty sure that things would
easily and quickly blow up if we had ranges like that.
-- 
Michal Hocko
SUSE Labs