From: Mike Rapoport <rppt@kernel.org>
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>,
"Matthew Wilcox (Oracle)" <willy@infradead.org>,
Oscar Salvador <osalvador@suse.de>,
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()
Date: Mon, 17 May 2021 09:43:08 +0300 [thread overview]
Message-ID: <YKIQfCjq13dSMHOs@kernel.org> (raw)
In-Reply-To: <20210514172247.176750-5-david@redhat.com>
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>
One nit below, otherwise
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
> include/linux/page-flags.h | 10 ++++++++++
> mm/util.c | 40 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 50 insertions(+)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index daed82744f4b..ea2df9a247b3 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -769,9 +769,19 @@ PAGE_TYPE_OPS(Buddy, buddy)
> * relies on this feature is aware that re-onlining the memory block will
> * require to re-set the pages PageOffline() and not giving them to the
> * buddy via online_page_callback_t.
> + *
> + * There are drivers that mark a page PageOffline() and do not expect any
Maybe "and expect there won't be any further access"...
> + * further access to page content. PFN walkers that read content of random
> + * pages should check PageOffline() and synchronize with such drivers using
> + * page_offline_freeze()/page_offline_thaw().
> */
> PAGE_TYPE_OPS(Offline, offline)
>
> +extern void page_offline_freeze(void);
> +extern void page_offline_thaw(void);
> +extern void page_offline_begin(void);
> +extern void page_offline_end(void);
> +
> /*
> * Marks pages in use as page tables.
> */
> diff --git a/mm/util.c b/mm/util.c
> index a8bf17f18a81..a034525e7ba2 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1010,3 +1010,43 @@ void mem_dump_obj(void *object)
> }
> EXPORT_SYMBOL_GPL(mem_dump_obj);
> #endif
> +
> +/*
> + * 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.
> + *
> + * 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().
> + *
> + * 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().
> + */
> +static DECLARE_RWSEM(page_offline_rwsem);
> +
> +void page_offline_freeze(void)
> +{
> + down_read(&page_offline_rwsem);
> +}
> +
> +void page_offline_thaw(void)
> +{
> + up_read(&page_offline_rwsem);
> +}
> +
> +void page_offline_begin(void)
> +{
> + down_write(&page_offline_rwsem);
> +}
> +EXPORT_SYMBOL(page_offline_begin);
> +
> +void page_offline_end(void)
> +{
> + up_write(&page_offline_rwsem);
> +}
> +EXPORT_SYMBOL(page_offline_end);
> --
> 2.31.1
>
--
Sincerely yours,
Mike.
next prev parent reply other threads:[~2021-05-17 6:43 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-14 17:22 [PATCH v2 0/6] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages David Hildenbrand
2021-05-14 17:22 ` [PATCH v2 1/6] fs/proc/kcore: drop KCORE_REMAP and KCORE_OTHER David Hildenbrand
2021-05-14 17:22 ` [PATCH v2 2/6] fs/proc/kcore: pfn_is_ram check only applies to KCORE_RAM David Hildenbrand
2021-05-14 17:22 ` [PATCH v2 3/6] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages David Hildenbrand
2021-05-25 8:09 ` Oscar Salvador
2021-05-14 17:22 ` [PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline() David Hildenbrand
2021-05-17 6:43 ` Mike Rapoport [this message]
2021-05-17 15:18 ` David Hildenbrand
2021-05-25 8:16 ` Oscar Salvador
2021-05-14 17:22 ` [PATCH v2 5/6] virtio-mem: use page_offline_(start|end) when " David Hildenbrand
2021-05-17 6:43 ` Mike Rapoport
2021-05-25 8:20 ` Oscar Salvador
2021-05-14 17:22 ` [PATCH v2 6/6] fs/proc/kcore: use page_offline_(freeze|thaw) David Hildenbrand
2021-05-17 6:44 ` Mike Rapoport
2021-05-25 8:21 ` Oscar Salvador
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