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From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <songmuchun@bytedance.com>,
	corbet@lwn.net, mike.kravetz@oracle.com, 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 10:43:40 +0100	[thread overview]
Message-ID: <eda50930-05b5-0ad9-2985-8b6328f92cec@redhat.com> (raw)
In-Reply-To: <20201120093912.GM3200@dhcp22.suse.cz>

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


  reply	other threads:[~2020-11-20  9:44 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 [this message]
2020-11-20 17:45         ` Mike Kravetz
2020-11-20 18:00           ` David Hildenbrand
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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