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Tsirkin" , linux-hyperv@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-mm@kvack.org, Wei Liu , Alex Shi , Stephen Hemminger , "Matthew Wilcox \(Oracle\)" , Steven Price , Alexey Dobriyan , Jiri Bohac , Haiyang Zhang , Oscar Salvador , Naoya Horiguchi , linux-fsdevel@vger.kernel.org, Andrew Morton , Roman Gushchin , Mike Rapoport , Mike Kravetz X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" 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|unfreeze) for synchronizing. page_offline_freeze()/page_offline_unfreeze() 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. Signed-off-by: David Hildenbrand --- include/linux/page-flags.h | 5 +++++ mm/util.c | 38 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 43 insertions(+) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index b8c56672a588..e3d00c72f459 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -767,6 +767,11 @@ PAGE_TYPE_OPS(Buddy, buddy) */ PAGE_TYPE_OPS(Offline, offline) +extern void page_offline_freeze(void); +extern void page_offline_unfreeze(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 54870226cea6..95395d4e4209 100644 --- a/mm/util.c +++ b/mm/util.c @@ -1013,3 +1013,41 @@ void mem_dump_obj(void *object) } pr_cont(" non-slab/vmalloc memory.\n"); } + +/* + * 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_unfreeze() 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_unfreeze(void) +{ + up_read(&page_offline_rwsem); +} + +void page_offline_begin(void) +{ + down_write(&page_offline_rwsem); +} + +void page_offline_end(void) +{ + up_write(&page_offline_rwsem); +} -- 2.30.2 _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization