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From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Rashmica Gupta <rashmica.g@gmail.com>,
	Oscar Salvador <osalvador@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Williams <dan.j.williams@intel.com>,
	pasha.tatashin@soleen.com, Jonathan.Cameron@huawei.com,
	anshuman.khandual@arm.com, Vlastimil Babka <vbabka@suse.cz>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory
Date: Thu, 1 Aug 2019 09:26:35 +0200	[thread overview]
Message-ID: <e654aa97-6ab1-4069-60e6-fc099539729e@redhat.com> (raw)
In-Reply-To: <20190801072430.GF11627@dhcp22.suse.cz>

On 01.08.19 09:24, Michal Hocko wrote:
> On Thu 01-08-19 09:18:47, David Hildenbrand wrote:
>> On 01.08.19 09:17, Michal Hocko wrote:
>>> On Thu 01-08-19 09:06:40, Rashmica Gupta wrote:
>>>> On Wed, 2019-07-31 at 14:08 +0200, Michal Hocko wrote:
>>>>> On Tue 02-07-19 18:52:01, Rashmica Gupta wrote:
>>>>> [...]
>>>>>>> 2) Why it was designed, what is the goal of the interface?
>>>>>>> 3) When it is supposed to be used?
>>>>>>>
>>>>>>>
>>>>>> There is a hardware debugging facility (htm) on some power chips.
>>>>>> To use
>>>>>> this you need a contiguous portion of memory for the output to be
>>>>>> dumped
>>>>>> to - and we obviously don't want this memory to be simultaneously
>>>>>> used by
>>>>>> the kernel.
>>>>>
>>>>> How much memory are we talking about here? Just curious.
>>>>
>>>> From what I've seen a couple of GB per node, so maybe 2-10GB total.
>>>
>>> OK, that is really a lot to keep around unused just in case the
>>> debugging is going to be used.
>>>
>>> I am still not sure the current approach of (ab)using memory hotplug is
>>> ideal. Sure there is some overlap but you shouldn't really need to
>>> offline the required memory range at all. All you need is to isolate the
>>> memory from any existing user and the page allocator. Have you checked
>>> alloc_contig_range?
>>>
>>
>> Rashmica mentioned somewhere in this thread that the virtual mapping
>> must not be in place, otherwise the HW might prefetch some of this
>> memory, leading to errors with memtrace (which checks that in HW).
> 
> Does anything prevent from unmapping the pfn range from the direct
> mapping?

I am not sure about the implications of having
pfn_valid()/pfn_present()/pfn_online() return true but accessing it
results in crashes. (suspend, kdump, whatever other technology touches
online memory)

(sounds more like a hack to me than just going ahead and
removing/readding the memory via a clean interface we have)

-- 

Thanks,

David / dhildenb

  reply	other threads:[~2019-08-01  7:26 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-25  7:52 [PATCH v2 0/5] Allocate memmap from hotadded memory Oscar Salvador
2019-06-25  7:52 ` [PATCH v2 1/5] drivers/base/memory: Remove unneeded check in remove_memory_block_devices Oscar Salvador
2019-06-25  8:01   ` David Hildenbrand
2019-06-25  8:03     ` David Hildenbrand
2019-06-25  8:09       ` Oscar Salvador
2019-06-25  8:27         ` David Hildenbrand
2019-06-25  7:52 ` [PATCH v2 2/5] mm,memory_hotplug: Introduce MHP_VMEMMAP_FLAGS Oscar Salvador
2019-06-25  8:31   ` David Hildenbrand
2019-07-24 20:11   ` Dan Williams
2019-07-24 20:11     ` Dan Williams
2019-07-24 21:36     ` osalvador
2019-07-25  9:27     ` Oscar Salvador
2019-07-25  9:30       ` David Hildenbrand
2019-07-25  9:40         ` Oscar Salvador
2019-07-25 10:04           ` David Hildenbrand
2019-07-25 10:13             ` Oscar Salvador
2019-07-25 10:15               ` David Hildenbrand
2019-06-25  7:52 ` [PATCH v2 3/5] mm,memory_hotplug: Introduce Vmemmap page helpers Oscar Salvador
2019-06-25 10:28   ` David Hildenbrand
2019-06-26  9:48     ` Oscar Salvador
2019-06-25  7:52 ` [PATCH v2 4/5] mm,memory_hotplug: allocate memmap from the added memory range for sparse-vmemmap Oscar Salvador
2019-06-25  8:49   ` David Hildenbrand
2019-06-26  8:13     ` Oscar Salvador
2019-06-26  8:15       ` David Hildenbrand
2019-06-26  8:17   ` Anshuman Khandual
2019-06-26  8:28     ` Oscar Salvador
2019-07-24 21:49   ` Dan Williams
2019-07-24 21:49     ` Dan Williams
2019-06-25  7:52 ` [PATCH v2 5/5] mm,memory_hotplug: Allow userspace to enable/disable vmemmap Oscar Salvador
2019-06-25  8:25 ` [PATCH v2 0/5] Allocate memmap from hotadded memory David Hildenbrand
2019-06-25  8:33   ` David Hildenbrand
2019-06-26  8:03   ` Oscar Salvador
2019-06-26  8:11     ` David Hildenbrand
2019-06-26  8:15       ` Oscar Salvador
2019-06-26  8:27         ` Oscar Salvador
2019-06-26  8:37           ` David Hildenbrand
2019-06-26  8:28         ` David Hildenbrand
2019-07-02  6:42           ` Rashmica Gupta
2019-07-02  6:42             ` Rashmica Gupta
2019-07-02  7:48             ` Oscar Salvador
2019-07-02  8:52               ` Rashmica Gupta
2019-07-10  1:14                 ` Rashmica Gupta
2019-07-10  1:14                   ` Rashmica Gupta
2019-07-31 12:08                 ` Michal Hocko
2019-07-31 23:06                   ` Rashmica Gupta
2019-07-31 23:06                     ` Rashmica Gupta
2019-08-01  7:17                     ` Michal Hocko
2019-08-01  7:18                       ` David Hildenbrand
2019-08-01  7:24                         ` Michal Hocko
2019-08-01  7:26                           ` David Hildenbrand [this message]
2019-08-01  7:31                             ` David Hildenbrand
2019-08-01  7:39                               ` Michal Hocko
2019-08-01  7:48                                 ` Michal Hocko
2019-08-01  9:18                                   ` David Hildenbrand
2019-08-01  7:34                             ` Michal Hocko
2019-08-01  7:50                               ` David Hildenbrand
2019-08-01  8:04                                 ` Michal Hocko
2019-07-16 12:28             ` David Hildenbrand
2019-07-29  5:42               ` Rashmica Gupta
2019-07-29  5:42                 ` Rashmica Gupta
2019-07-29  8:06                 ` David Hildenbrand
2019-07-30  7:08                   ` Rashmica Gupta
2019-07-30  7:08                     ` Rashmica Gupta
2019-07-31  2:21                   ` Rashmica Gupta
2019-07-31  2:21                     ` Rashmica Gupta
2019-07-31  9:39                     ` David Hildenbrand

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