On 05.08.21 09:48, Philippe Mathieu-Daudé wrote:
On 7/30/21 10:52 AM, David Hildenbrand wrote:Currently, when someone (i.e., the VM) accesses discarded parts inside a RAMBlock with a RamDiscardManager managing the corresponding mapped memory region, postcopy will request migration of the corresponding page from the source. The source, however, will never answer, because it refuses to migrate such pages with undefined content ("logically unplugged"): the pages are never dirty, and get_queued_page() will consequently skip processing these postcopy requests.Especially reading discarded ("logically unplugged") ranges is supposed to work in some setups (for example with current virtio-mem), although it barely ever happens: still, not placing a page would currently stall the VM, as it cannot make forward progress. Let's check the state via the RamDiscardManager (the state e.g., of virtio-mem is migrated during precopy) and avoid sending a request that will never get answered. Place a fresh zero page instead to keep the VM working. This is the same behavior that would happen automatically without userfaultfd being active, when accessing virtual memory regions without populated pages -- "populate on demand". For now, there are valid cases (as documented in the virtio-mem spec) where a VM might read discarded memory; in the future, we will disallow that. Then, we might want to handle that case differently, e.g., warning the user that the VM seems to be mis-behaving. Signed-off-by: David Hildenbrand <da...@redhat.com> --- migration/postcopy-ram.c | 31 +++++++++++++++++++++++++++---- migration/ram.c | 21 +++++++++++++++++++++ migration/ram.h | 1 + 3 files changed, 49 insertions(+), 4 deletions(-) diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c index 2e9697bdd2..38cdfc09c3 100644 --- a/migration/postcopy-ram.c +++ b/migration/postcopy-ram.c @@ -671,6 +671,29 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, return ret; }+static int postcopy_request_page(MigrationIncomingState *mis, RAMBlock *rb,+ ram_addr_t start, uint64_t haddr) +{ + void *aligned = (void *)(uintptr_t)(haddr & -qemu_ram_pagesize(rb));void *aligned = QEMU_ALIGN_PTR_DOWN(haddr, qemu_ram_pagesize(rb)));
Does not compile as haddr is not a pointer. void *aligned = QEMU_ALIGN_PTR_DOWN((void *)haddr, qemu_ram_pagesize(rb))); should work. I can also add a patch to adjust a similar call in migrate_send_rp_req_pages()! Thanks! -- Thanks, David / dhildenb
