David Hildenbrand
2021-May-17 15:18 UTC
[PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()
On 17.05.21 08:43, Mike Rapoport wrote:> 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 at suse.com> >> Signed-off-by: David Hildenbrand <david at redhat.com> > > One nit below, otherwise > > Reviewed-by: Mike Rapoport <rppt at 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"... >Thanks, makes sense. I'll wait a bit before I resend. -- Thanks, David / dhildenb