linux-kernel - Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

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Date:   Fri, 26 Mar 2021 15:53:41 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

On 26.03.21 15:38, Michal Hocko wrote:
> On Fri 26-03-21 09:52:58, David Hildenbrand wrote:
> [...]
>> Something else to note:
>>
>>
>> We'll not call the memory notifier (e.g., MEM_ONLINE) for the vmemmap. The
>> result is that
>>
>> 1. We won't allocate extended struct pages for the range. Don't think this
>> is really problematic (pages are never allocated/freed, so I guess we don't
>> care - like ZONE_DEVICE code).
> 
> Agreed. I do not think we need them. Future might disagree but let's
> handle it when we have a clear demand.
> 
>> 2. We won't allocate kasan shadow memory. We most probably have to do it
>> explicitly via kasan_add_zero_shadow()/kasan_remove_zero_shadow(), see
>> mm/memremap.c:pagemap_range()
> 
> I think this is similar to the above. Does kasan has to know about
> memory which will never be used for anything?

IIRC, kasan will track read/writes to the vmemmap as well. So it could 
theoretically detect if we read from the vmemmap before writing 
(initializing) it IIUC.

This is also why mm/memremap.c does a kasan_add_zero_shadow() before the 
move_pfn_range_to_zone()->memmap_init_range() for the whole region, 
including altmap space.

Now, I am no expert on KASAN, what would happen in case we have access 
to non-tracked memory.

commit 0207df4fa1a869281ddbf72db6203dbf036b3e1a
Author: Andrey Ryabinin <ryabinin.a.a@...il.com>
Date:   Fri Aug 17 15:47:04 2018 -0700

     kernel/memremap, kasan: make ZONE_DEVICE with work with KASAN

indicates that kasan will crash the system on "non-existent shadow memory"

> 
>> Further a locking rework might be necessary. We hold the device hotplug
>> lock, but not the memory hotplug lock. E.g., for get_online_mems(). Might
>> have to move that out online_pages.
> 
> Could you be more explicit why this locking is needed? What it would
> protect from for vmemmap pages?
> 

One example is in mm/kmemleak.c:kmemleak_scan(), where we scan the 
vmemmap for pointers. We don't want the vmemmap to get unmapped while we 
are working on it (-> fault).

-- 
Thanks,

David / dhildenb