On 08.05.2012, at 12:24, Paul Mackerras wrote: > From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> > > The H_REGISTER_VPA hcall implementation in HV Power KVM needs to pin some > guest memory pages into host memory so that they can be safely accessed > from usermode. It does this used get_user_pages_fast(). When the VPA is > unregistered, or the VCPUs are cleaned up, these pages are released using > put_page(). > > However, the get_user_pages() is invoked on the specific memory are of the > VPA which could lie within hugepages. In case the pinned page is huge, > we explicitly find the head page of the compound page before calling > put_page() on it. > > At least with the latest kernel, this is not correct. put_page() already > handles finding the correct head page of a compound, and also deals with > various counts on the individual tail page which are important for > transparent huge pages. We don't support transparent hugepages on Power, > but even so, bypassing this count maintenance can lead (when the VM ends) > to a hugepage being released back to the pool with a non-zero mapcount on > one of the tail pages. This can then lead to a bad_page() when the page > is released from the hugepage pool. > > This removes the explicit compound_head() call to correct this bug. > > Signed-off-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx> Acked-by: Alexander Graf <agraf@xxxxxxx> Avi, could you please make sure this makes the next 3.4-rc or -stable? Thanks! Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
