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Date:   Fri, 20 Nov 2020 10:43:40 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        mike.kravetz@...cle.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page

On 20.11.20 10:39, Michal Hocko wrote:
> On Fri 20-11-20 10:27:05, David Hildenbrand wrote:
>> On 20.11.20 09:42, Michal Hocko wrote:
>>> On Fri 20-11-20 14:43:04, Muchun Song wrote:
>>> [...]
>>>
>>> Thanks for improving the cover letter and providing some numbers. I have
>>> only glanced through the patchset because I didn't really have more time
>>> to dive depply into them.
>>>
>>> Overall it looks promissing. To summarize. I would prefer to not have
>>> the feature enablement controlled by compile time option and the kernel
>>> command line option should be opt-in. I also do not like that freeing
>>> the pool can trigger the oom killer or even shut the system down if no
>>> oom victim is eligible.
>>>
>>> One thing that I didn't really get to think hard about is what is the
>>> effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be
>>> invalid when racing with the split. How do we enforce that this won't
>>> blow up?
>>
>> I have the same concerns - the sections are online the whole time and
>> anybody with pfn_to_online_page() can grab them
>>
>> I think we have similar issues with memory offlining when removing the
>> vmemmap, it's just very hard to trigger and we can easily protect by
>> grabbing the memhotplug lock.
> 
> I am not sure we can/want to span memory hotplug locking out to all pfn
> walkers. But you are right that the underlying problem is similar but
> much harder to trigger because vmemmaps are only removed when the
> physical memory is hotremoved and that happens very seldom. Maybe it
> will happen more with virtualization usecases. But this work makes it
> even more tricky. If a pfn walker races with a hotremove then it would
> just blow up when accessing the unmapped physical address space. For
> this feature a pfn walker would just grab a real struct page re-used for
> some unpredictable use under its feet. Any failure would be silent and
> hard to debug.

Right, we don't want the memory hotplug locking, thus discussions 
regarding rcu. Luckily, for now I never saw a BUG report regarding this 
- maybe because the time between memory offlining (offline_pages()) and 
memory/vmemmap getting removed (try_remove_memory()) is just too long. 
Someone would have to sleep after pfn_to_online_page() for quite a while 
to trigger it.

> 
> [...]
>> To keep things easy, maybe simply never allow to free these hugetlb pages
>> again for now? If they were reserved during boot and the vmemmap condensed,
>> then just let them stick around for all eternity.
> 
> Not sure I understand. Do you propose to only free those vmemmap pages
> when the pool is initialized during boot time and never allow to free
> them up? That would certainly make it safer and maybe even simpler wrt
> implementation.

Exactly, let's keep it simple for now. I guess most use cases of this 
(virtualization, databases, ...) will allocate hugepages during boot and 
never free them.

-- 
Thanks,

David / dhildenb

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