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David Hildenbrand
8284045193
A driver might set a page logically offline -- PageOffline() -- and turn the page inaccessible in the hypervisor; after that, access to page content can be fatal. One example is virtio-mem; while unplugged memory -- marked as PageOffline() can currently be read in the hypervisor, this will no longer be the case in the future; for example, when having a virtio-mem device backed by huge pages in the hypervisor. Some special PFN walkers -- i.e., /proc/kcore -- read content of random pages after checking PageOffline(); however, these PFN walkers can race with drivers that set PageOffline(). Let's introduce page_offline_(begin|end|freeze|thaw) for synchronizing. page_offline_freeze()/page_offline_thaw() allows for a subsystem to synchronize with such drivers, achieving that a page cannot be set PageOffline() while frozen. page_offline_begin()/page_offline_end() is used by drivers that care about such races when setting a page PageOffline(). For simplicity, use a rwsem for now; neither drivers nor users are performance sensitive. Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: David Hildenbrand <[email protected]> Acked-by: Michal Hocko <[email protected]> Reviewed-by: Mike Rapoport <[email protected]> Reviewed-by: Oscar Salvador <[email protected]> Cc: Aili Yao <[email protected]> Cc: Alexey Dobriyan <[email protected]> Cc: Alex Shi <[email protected]> Cc: Haiyang Zhang <[email protected]> Cc: Jason Wang <[email protected]> Cc: Jiri Bohac <[email protected]> Cc: "K. Y. Srinivasan" <[email protected]> Cc: "Matthew Wilcox (Oracle)" <[email protected]> Cc: "Michael S. Tsirkin" <[email protected]> Cc: Mike Kravetz <[email protected]> Cc: Naoya Horiguchi <[email protected]> Cc: Roman Gushchin <[email protected]> Cc: Stephen Hemminger <[email protected]> Cc: Steven Price <[email protected]> Cc: Wei Liu <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>