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Date:   Tue, 25 May 2021 10:16:26 +0200
From:   Oscar Salvador <osalvador@suse.de>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Steven Price <steven.price@arm.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Aili Yao <yaoaili@kingsoft.com>, Jiri Bohac <jbohac@suse.cz>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        linux-hyperv@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v2 4/6] mm: introduce
 page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()
Message-ID: <20210525081626.GB3300@linux>
References: <20210514172247.176750-1-david@redhat.com>
 <20210514172247.176750-5-david@redhat.com>
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On Fri, May 14, 2021 at 07:22:45PM +0200, David Hildenbrand wrote:
> 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.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

-- 
Oscar Salvador
SUSE L3