Date:   Fri, 20 Nov 2020 19:00:15 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        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 18:45, Mike Kravetz wrote:
> On 11/20/20 1:43 AM, David Hildenbrand wrote:
>> 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.
> 
> Not sure if I agree with that last statement.  Database and virtualization
> use cases from my employer allocate allocate hugetlb pages after boot.  It
> is shortly after boot, but still not from boot/kernel command line.

Right, but the ones that care about this optimization for now could be 
converted, I assume? I mean we are talking about "opt-in" from 
sysadmins, so requiring to specify a different cmdline parameter does 
not sound to weird to me. And it should simplify a first version quite a 
lot.

The more I think about this, the more I believe doing these vmemmap 
modifications after boot are very dangerous.

> 
> Somewhat related, but not exactly addressing this issue ...
> 
> One idea discussed in a previous patch set was to disable PMD/huge page
> mapping of vmemmap if this feature was enabled.  This would eliminate a bunch
> of the complex code doing page table manipulation.  It does not address
> the issue of struct page pages going away which is being discussed here,
> but it could be a way to simply the first version of this code.  If this
> is going to be an 'opt in' feature as previously suggested, then eliminating
> the  PMD/huge page vmemmap mapping may be acceptable.  My guess is that
> sysadmins would only 'opt in' if they expect most of system memory to be used
> by hugetlb pages.  We certainly have database and virtualization use cases
> where this is true.

It sounds like a hack to me, which does not fully solve the problem. But 
yeah, it's a simplification.

-- 
Thanks,

David / dhildenb

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