From: David Hildenbrand <david@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.com>, Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <songmuchun@bytedance.com>,
corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com,
bp@alien8.de, x86@kernel.org, hpa@zytor.com,
dave.hansen@linux.intel.com, luto@kernel.org,
peterz@infradead.org, viro@zeniv.linux.org.uk,
akpm@linux-foundation.org, paulmck@kernel.org,
mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com,
rdunlap@infradead.org, oneukum@suse.com,
anshuman.khandual@arm.com, jroedel@suse.de,
almasrymina@google.com, rientjes@google.com, willy@infradead.org,
osalvador@suse.de, song.bao.hua@hisilicon.com,
duanxiongchun@bytedance.com, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page
Date: Fri, 20 Nov 2020 19:00:15 +0100 [thread overview]
Message-ID: <af95bcad-80dd-d2a4-0178-b9d2869e97cf@redhat.com> (raw)
In-Reply-To: <55e53264-a07a-a3ec-4253-e72c718b4ee6@oracle.com>
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
next prev parent reply other threads:[~2020-11-20 18:00 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-20 6:43 [PATCH v5 00/21] Free some vmemmap pages of hugetlb page Muchun Song
2020-11-20 6:43 ` [PATCH v5 01/21] mm/memory_hotplug: Move bootmem info registration API to bootmem_info.c Muchun Song
2020-11-20 6:43 ` [PATCH v5 02/21] mm/memory_hotplug: Move {get,put}_page_bootmem() " Muchun Song
2020-11-20 6:43 ` [PATCH v5 03/21] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP Muchun Song
2020-11-20 7:49 ` Michal Hocko
2020-11-20 8:35 ` [External] " Muchun Song
2020-11-20 8:47 ` Michal Hocko
2020-11-20 8:53 ` Muchun Song
2020-11-20 6:43 ` [PATCH v5 04/21] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate Muchun Song
2020-11-20 6:43 ` [PATCH v5 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers Muchun Song
2020-11-20 6:43 ` [PATCH v5 06/21] mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() Muchun Song
2020-11-20 6:43 ` [PATCH v5 07/21] mm/bootmem_info: Combine bootmem info and type into page->freelist Muchun Song
2020-11-20 6:43 ` [PATCH v5 08/21] mm/hugetlb: Initialize page table lock for vmemmap Muchun Song
2020-11-20 6:43 ` [PATCH v5 09/21] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page Muchun Song
2020-11-20 6:43 ` [PATCH v5 10/21] mm/hugetlb: Defer freeing of hugetlb pages Muchun Song
2020-11-20 6:43 ` [PATCH v5 11/21] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page Muchun Song
2020-11-20 8:11 ` Michal Hocko
2020-11-20 8:51 ` [External] " Muchun Song
2020-11-20 9:28 ` Michal Hocko
2020-11-20 9:37 ` Muchun Song
2020-11-20 11:10 ` Michal Hocko
2020-11-20 11:56 ` Muchun Song
2020-11-20 6:43 ` [PATCH v5 12/21] mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper Muchun Song
2020-11-20 6:43 ` [PATCH v5 13/21] mm/hugetlb: Use PG_slab to indicate split pmd Muchun Song
2020-11-20 8:16 ` Michal Hocko
2020-11-20 9:30 ` [External] " Muchun Song
2020-11-23 7:48 ` Michal Hocko
2020-11-23 8:01 ` Muchun Song
2020-11-20 6:43 ` [PATCH v5 14/21] mm/hugetlb: Support freeing vmemmap pages of gigantic page Muchun Song
2020-11-20 6:43 ` [PATCH v5 15/21] mm/hugetlb: Set the PageHWPoison to the raw error page Muchun Song
2020-11-20 8:19 ` Michal Hocko
2020-11-20 10:32 ` [External] " Muchun Song
2020-11-20 6:43 ` [PATCH v5 16/21] mm/hugetlb: Flush work when dissolving hugetlb page Muchun Song
2020-11-20 8:20 ` Michal Hocko
2020-11-20 6:43 ` [PATCH v5 17/21] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap Muchun Song
2020-11-20 8:22 ` Michal Hocko
2020-11-20 10:39 ` [External] " Muchun Song
2020-11-20 6:43 ` [PATCH v5 18/21] mm/hugetlb: Merge pte to huge pmd only for gigantic page Muchun Song
2020-11-20 8:23 ` Michal Hocko
2020-11-20 10:41 ` [External] " Muchun Song
2020-11-20 6:43 ` [PATCH v5 19/21] mm/hugetlb: Gather discrete indexes of tail page Muchun Song
2020-11-20 6:43 ` [PATCH v5 20/21] mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct page Muchun Song
2020-11-20 6:43 ` [PATCH v5 21/21] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two Muchun Song
2020-11-20 8:25 ` Michal Hocko
2020-11-20 9:15 ` David Hildenbrand
2020-11-22 13:30 ` Mike Rapoport
2020-11-22 19:00 ` Matthew Wilcox
2020-11-23 3:14 ` [External] " Muchun Song
2020-11-20 9:16 ` David Hildenbrand
2020-11-20 10:42 ` [External] " Muchun Song
2020-11-20 8:42 ` [PATCH v5 00/21] Free some vmemmap pages of hugetlb page Michal Hocko
2020-11-20 9:27 ` David Hildenbrand
2020-11-20 9:39 ` Michal Hocko
2020-11-20 9:43 ` David Hildenbrand
2020-11-20 17:45 ` Mike Kravetz
2020-11-20 18:00 ` David Hildenbrand [this message]
2020-11-22 7:29 ` [External] " Muchun Song
2020-11-23 7:38 ` Michal Hocko
2020-11-23 21:52 ` Mike Kravetz
2020-11-23 22:01 ` Matthew Wilcox
2020-11-20 12:40 ` [External] " Muchun Song
2020-11-20 13:11 ` Michal Hocko
2020-11-20 15:44 ` Muchun Song
2020-11-23 7:40 ` Michal Hocko
2020-11-23 8:53 ` Muchun Song
2020-11-23 9:43 ` Michal Hocko
2020-11-23 10:36 ` Muchun Song
2020-11-23 10:42 ` Michal Hocko
2020-11-23 11:16 ` Muchun Song
2020-11-23 11:32 ` Michal Hocko
2020-11-23 12:07 ` Muchun Song
2020-11-23 12:18 ` Michal Hocko
2020-11-23 12:40 ` Muchun Song
2020-11-23 12:48 ` Michal Hocko
2020-11-23 12:45 ` Matthew Wilcox
2020-11-23 13:05 ` Muchun Song
2020-11-23 13:13 ` Michal Hocko
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