linux-kernel - Re: [PATCH v3 10/11] mm/memory_hotplug: Make unregister_memory_block_under

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Message-ID: <d9e63646-f81d-c349-a54c-e17cdccb0760@redhat.com>
Date:   Tue, 16 Jul 2019 13:08:56 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        akpm@...ux-foundation.org, Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Mark Brown <broonie@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v3 10/11] mm/memory_hotplug: Make
 unregister_memory_block_under_nodes() never fail

On 16.07.19 10:46, Oscar Salvador wrote:
> On Mon, Jul 15, 2019 at 01:10:33PM +0200, David Hildenbrand wrote:
>> On 01.07.19 12:27, Michal Hocko wrote:
>>> On Mon 01-07-19 11:36:44, Oscar Salvador wrote:
>>>> On Mon, Jul 01, 2019 at 10:51:44AM +0200, Michal Hocko wrote:
>>>>> Yeah, we do not allow to offline multi zone (node) ranges so the current
>>>>> code seems to be over engineered.
>>>>>
>>>>> Anyway, I am wondering why do we have to strictly check for already
>>>>> removed nodes links. Is the sysfs code going to complain we we try to
>>>>> remove again?
>>>>
>>>> No, sysfs will silently "fail" if the symlink has already been removed.
>>>> At least that is what I saw last time I played with it.
>>>>
>>>> I guess the question is what if sysfs handling changes in the future
>>>> and starts dropping warnings when trying to remove a symlink is not there.
>>>> Maybe that is unlikely to happen?
>>>
>>> And maybe we handle it then rather than have a static allocation that
>>> everybody with hotremove configured has to pay for.
>>>
>>
>> So what's the suggestion? Dropping the nodemask_t completely and calling
>> sysfs_remove_link() on already potentially removed links?
>>
>> Of course, we can also just use mem_blk->nid and rest assured that it
>> will never be called for memory blocks belonging to multiple nodes.
> 
> Hi David,
> 
> While it is easy to construct a scenario where a memblock belongs to multiple
> nodes, I have to confess that I yet have not seen that in a real-world scenario.
> 
> Given said that, I think that the less risky way is to just drop the nodemask_t
> and do not care about calling sysfs_remove_link() for already removed links.
> As I said, sysfs_remove_link() will silently fail when it fails to find the
> symlink, so I do not think it is a big deal.
> 
> 

As far as I can tell we

a) don't allow offlining of memory that belongs to multiple nodes
already (as pointed out by Michael recently)

b) users cannot add memory blocks that belong to multiple nodes via
add_memory()

So I don't see a way how remove_memory() (and even offline_pages())
could ever succeed on such memory blocks.

I think it should be fine to limit it to one node here. (if not, I guess
we would have a different BUG that would actually allow to remove such
memory blocks)

-- 

Thanks,

David / dhildenb