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fs/proc/kcore: don't read offline sections, logically offline pages a…
…nd hwpoisoned pages Let's avoid reading: 1) Offline memory sections: the content of offline memory sections is stale as the memory is effectively unused by the kernel. On s390x with standby memory, offline memory sections (belonging to offline storage increments) are not accessible. With virtio-mem and the hyper-v balloon, we can have unavailable memory chunks that should not be accessed inside offline memory sections. Last but not least, offline memory sections might contain hwpoisoned pages which we can no longer identify because the memmap is stale. 2) PG_offline pages: logically offline pages that are documented as "The content of these pages is effectively stale. Such pages should not be touched (read/write/dump/save) except by their owner.". Examples include pages inflated in a balloon or unavailble memory ranges inside hotplugged memory sections with virtio-mem or the hyper-v balloon. 3) PG_hwpoison pages: Reading pages marked as hwpoisoned can be fatal. As documented: "Accessing is not safe since it may cause another machine check. Don't touch!" Introduce is_page_hwpoison(), adding a comment that it is inherently racy but best we can really do. Reading /proc/kcore now performs similar checks as when reading /proc/vmcore for kdump via makedumpfile: problematic pages are exclude. It's also similar to hibernation code, however, we don't skip hwpoisoned pages when processing pages in kernel/power/snapshot.c:saveable_page() yet. Note 1: we can race against memory offlining code, especially memory going offline and getting unplugged: however, we will properly tear down the identity mapping and handle faults gracefully when accessing this memory from kcore code. Note 2: we can race against drivers setting PageOffline() and turning memory inaccessible in the hypervisor. We'll handle this in a follow-up patch. Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: David Hildenbrand <[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: Michal Hocko <[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]>
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