linux-kernel - Re: [PATCH v2 5/6] virtio-mem: use page_offline

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Date:   Tue, 25 May 2021 10:20:36 +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
Subject: Re: [PATCH v2 5/6] virtio-mem: use page_offline_(start|end) when
 setting PageOffline()

On Fri, May 14, 2021 at 07:22:46PM +0200, David Hildenbrand wrote:
> Let's properly use page_offline_(start|end) to synchronize setting
> PageOffline(), so we won't have valid page access to unplugged memory
> regions from /proc/kcore.
> 
> Existing balloon implementations usually allow reading inflated memory;
> doing so might result in unnecessary overhead in the hypervisor, which
> is currently the case with virtio-mem.
> 
> For future virtio-mem use cases, it will be different when using shmem,
> huge pages, !anonymous private mappings, ... as backing storage for a VM.
> virtio-mem unplugged memory must no longer be accessed and access might
> result in undefined behavior. There will be a virtio spec extension to
> document this change, including a new feature flag indicating the
> changed behavior. We really don't want to race against PFN walkers
> reading random page content.
> 
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

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


-- 
Oscar Salvador
SUSE L3