# Re: \[PATCH v2 3/6\] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages

- From: Oscar Salvador \<osalvador@suse.de\>
- Date: 2021-05-25 08:11:20 
- List: linux-hyperv
- Page: https://ratatoskr.run/linux-hyperv/2021/05/206524
- Thread: https://ratatoskr.run/linux-hyperv/2021/05/206036/t.md

```text
On Fri, May 14, 2021 at 07:22:44PM +0200, David Hildenbrand wrote:
> 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.
> 
> Reviewed-by: Mike Rapoport <redacted>
> Signed-off-by: David Hildenbrand <redacted>

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE L3
```