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@xxxxxxxxx> > 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
