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: <20190814160220.GF17933@dhcp22.suse.cz>
Date:   Wed, 14 Aug 2019 18:02:20 +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 16:28:21, David Hildenbrand wrote:
> On 14.08.19 16:17, 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.
> 
> So, while I agree that "not a power of two" is rare, are you sure we
> will only have holes that are aligned to 4MB (especially on x86)?

No I am not saying that because I do not know. I am not aware of any
such HW but that doesn't really mean anything. But I would like to see
an example and have it archived in the changelog before we make the code
more complicated.
-- 
Michal Hocko
SUSE Labs