linux-kernel - Re: [PATCH v1] drivers/base/node.c: Simplify unregister_memory_block_under

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Message-ID: <20190719090942.GQ30461@dhcp22.suse.cz>
Date:   Fri, 19 Jul 2019 11:09:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] drivers/base/node.c: Simplify
 unregister_memory_block_under_nodes()

On Fri 19-07-19 10:48:19, David Hildenbrand wrote:
> On 19.07.19 10:42, Michal Hocko wrote:
> > On Thu 18-07-19 16:22:39, David Hildenbrand wrote:
> >> We don't allow to offline memory block devices that belong to multiple
> >> numa nodes. Therefore, such devices can never get removed. It is
> >> sufficient to process a single node when removing the memory block.
> >>
> >> Remember for each memory block if it belongs to no, a single, or mixed
> >> nodes, so we can use that information to skip unregistering or print a
> >> warning (essentially a safety net to catch BUGs).
> > 
> > I do not really like NUMA_NO_NODE - 1 thing. This is yet another invalid
> > node that is magic. Why should we even care? In other words why is this
> > patch an improvement?
> 
> I mean we can of course go ahead and drop the "NUMA_NO_NODE - 1" thingy
> from the patch. A memory block with multiple nodes would (as of now)
> only indicate one of the nodes.

Yes and that seemed to work reasonably well so far. Sure there is a
potential confusion but platforms with interleaved nodes are rare enough
to somebody to even notice so far.

> Then there is simply no way to WARN_ON_ONCE() in case unexpected things
> would happen. (I mean it really shouldn't happen or we have a BUG
> somewhere else)

I do not really see much point to warn here. What can user potentially
do?

> Alternative: Add "bool mixed_nids;" to "struct memory block".

That would be certainly possible but do we actually care?
-- 
Michal Hocko
SUSE Labs