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SubjectRe: [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@gmail.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

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