Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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 09:50:29 +0200	[thread overview]
Message-ID: <7c12f0b1-61a5-ed6f-2c64-4058e47860a3@redhat.com> (raw)
In-Reply-To: <20190801073407.GG11627@dhcp22.suse.cz>

On 01.08.19 09:34, Michal Hocko wrote:
> On Thu 01-08-19 09:26:35, David Hildenbrand wrote:
>> 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)
> 
> If those pages are marked as Reserved then nobody should be touching
> them anyway.

Which is not true as I remember we already discussed - I even documented
what PG_reserved can mean after that discussion in page-flags.h (e.g.,
memmap of boot memory) - that's why we introduced PG_offline after all.

-- 

Thanks,

David / dhildenb