Date:   Thu, 20 Jul 2017 08:16:49 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/9] mm, page_alloc: remove stop_machine from
 build_all_zonelists

On 07/14/2017 01:45 PM, Michal Hocko wrote:
> On Fri 14-07-17 13:43:21, Michal Hocko wrote:
>> On Fri 14-07-17 13:29:14, Vlastimil Babka wrote:
>>> On 07/14/2017 10:00 AM, Michal Hocko wrote:
>>>> From: Michal Hocko <mhocko@...e.com>
>>>>
>>>> build_all_zonelists has been (ab)using stop_machine to make sure that
>>>> zonelists do not change while somebody is looking at them. This is
>>>> is just a gross hack because a) it complicates the context from which
>>>> we can call build_all_zonelists (see 3f906ba23689 ("mm/memory-hotplug:
>>>> switch locking to a percpu rwsem")) and b) is is not really necessary
>>>> especially after "mm, page_alloc: simplify zonelist initialization".
>>>>
>>>> Updates of the zonelists happen very seldom, basically only when a zone
>>>> becomes populated during memory online or when it loses all the memory
>>>> during offline. A racing iteration over zonelists could either miss a
>>>> zone or try to work on one zone twice. Both of these are something we
>>>> can live with occasionally because there will always be at least one
>>>> zone visible so we are not likely to fail allocation too easily for
>>>> example.
>>>
>>> Given the experience with with cpusets and mempolicies, I would rather
>>> avoid the risk of allocation not seeing the only zone(s) that are
>>> allowed by its nodemask, and triggering premature OOM.
>>
>> I would argue, those are a different beast because they are directly
>> under control of not fully priviledged user and change between the empty
>> nodemask and cpusets very often. For this one to trigger we
>> would have to online/offline the last memory block in the zone very
>> often and that doesn't resemble a sensible usecase even remotely.

OK.

>>> So maybe the
>>> updates could be done in a way to avoid that, e.g. first append a copy
>>> of the old zonelist to the end, then overwrite and terminate with NULL.
>>> But if this requires any barriers or something similar on the iteration
>>> site, which is performance critical, then it's bad.
>>> Maybe a seqcount, that the iteration side only starts checking in the
>>> slowpath? Like we have with cpusets now.
>>> I know that Mel noted that stop_machine() also never had such guarantees
>>> to prevent this, but it could have made the chances smaller.
>>
>> I think we can come up with some scheme but is this really worth it
>> considering how unlikely the whole thing is? Well, if somebody hits a
>> premature OOM killer or allocations failures it would have to be along
>> with a heavy memory hotplug operations and then it would be quite easy
>> to spot what is going on and try to fix it. I would rather not
>> overcomplicate it, to be honest.

Fine, we can always add it later.

> And one more thing, Mel has already brought this up in his response.
> stop_machine haven't is very roughly same strenght wrt. double zone
> visit or a missed zone because we do not restart zonelist iteration.

I know, that's why I wrote "I know that Mel noted that stop_machine()
also never had such guarantees to prevent this, but it could have made
the chances smaller." But I don't have any good proof that your patch is
indeed making things worse, so let's apply and see...

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