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

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Message-ID: <20210525080922.GA3300@linux>
Date:   Tue, 25 May 2021 10:09:27 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Steven Price <steven.price@....com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Aili Yao <yaoaili@...gsoft.com>, Jiri Bohac <jbohac@...e.cz>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-hyperv@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v2 3/6] fs/proc/kcore: don't read offline sections,
 logically offline pages and hwpoisoned pages

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 <rppt@...ux.ibm.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

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


-- 
Oscar Salvador
SUSE L3