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 16:31:05 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.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 Fri 26-03-21 15:53:41, David Hildenbrand wrote:
> On 26.03.21 15:38, Michal Hocko wrote:
> > On Fri 26-03-21 09:52:58, David Hildenbrand wrote:
[...]
> > > 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"

Interesting. Thanks for the pointer.

> > > 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).
 
Hmm, but they are not going away during offline. They just have a less
defined state. Or what exactly do you mean by unmapped?
-- 
Michal Hocko
SUSE Labs