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

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Date:   Thu, 1 Aug 2019 11:18:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Rashmica Gupta <rashmica.g@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        pasha.tatashin@...een.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

On 01.08.19 09:48, Michal Hocko wrote:
> On Thu 01-08-19 09:39:57, Michal Hocko wrote:
>> On Thu 01-08-19 09:31:09, David Hildenbrand wrote:
>>> On 01.08.19 09:26, David Hildenbrand wrote:
>> [...]
>>>> 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)
>>>
>>> (oneidea: we could of course go ahead and mark the pages PG_offline
>>> before unmapping the pfn range to work around these issues)
>>
>> PG_reserved and an elevated reference count should be enough to drive
>> any pfn walker out. Pfn walkers shouldn't touch any page unless they
>> know and recognize their type.
> 
> Btw. this shouldn't be much different from DEBUG_PAGE_ALLOC in
> principle. The memory is valid, but not mapped to the kernel virtual
> space. Nobody should be really touching it anyway.
> 

I guess that could work (I am happy with anything that gets rid of
offline_pages()/device_online() here :D ).

So for each node, alloc_contig_range() (if I remember correctly, all
pages in the range have to be in the same zone), set them PG_reserved (+
maybe something else, we'll have to see). Then, unmap them.

The reverse when freeing the memory. Guess this should leave the current
user space interface unmodified.

I can see that guard pages use a special page type (PageGuard), not sure
if something like that is really required. We'll have to see.

-- 

Thanks,

David / dhildenb