On 21.02.20 09:35, David Hildenbrand wrote: > On 20.02.20 21:54, Peter Xu wrote: >> On Wed, Feb 19, 2020 at 05:17:22PM +0100, David Hildenbrand wrote: >>> Resizing while migrating is dangerous and does not work as expected. >>> The whole migration code works on the usable_length of ram blocks and does >>> not expect this to change at random points in time. >>> >>> In the case of postcopy, relying on used_length is racy as soon as the >>> guest is running. Also, when used_length changes we might leave the >>> uffd handler registered for some memory regions, reject valid pages >>> when migrating and fail when sending the recv bitmap to the source. >>> >>> Resizing can be trigger *after* (but not during) a reset in >>> ACPI code by the guest >>> - hw/arm/virt-acpi-build.c:acpi_ram_update() >>> - hw/i386/acpi-build.c:acpi_ram_update() >>> >>> Let's remember the original used_length in a separate variable and >>> use it in relevant postcopy code. Make sure to update it when we resize >>> during precopy, when synchronizing the RAM block sizes with the source. >>> >>> Cc: "Dr. David Alan Gilbert" <dgilb...@redhat.com> >>> Cc: Juan Quintela <quint...@redhat.com> >>> Cc: Eduardo Habkost <ehabk...@redhat.com> >>> Cc: Paolo Bonzini <pbonz...@redhat.com> >>> Cc: Igor Mammedov <imamm...@redhat.com> >>> Cc: "Michael S. Tsirkin" <m...@redhat.com> >>> Cc: Richard Henderson <richard.hender...@linaro.org> >>> Cc: Shannon Zhao <shannon.z...@linaro.org> >>> Cc: Alex Bennée <alex.ben...@linaro.org> >>> Cc: Peter Xu <pet...@redhat.com> >>> Signed-off-by: David Hildenbrand <da...@redhat.com> >>> --- >>> include/exec/ramblock.h | 9 +++++++++ >>> migration/postcopy-ram.c | 15 ++++++++++++--- >>> migration/ram.c | 11 +++++++++-- >>> 3 files changed, 30 insertions(+), 5 deletions(-) >>> >>> diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h >>> index 07d50864d8..0e9e9b346b 100644 >>> --- a/include/exec/ramblock.h >>> +++ b/include/exec/ramblock.h >>> @@ -59,6 +59,15 @@ struct RAMBlock { >>> */ >>> unsigned long *clear_bmap; >>> uint8_t clear_bmap_shift; >>> + >>> + /* >>> + * RAM block used_length before the guest started running while >>> postcopy >>> + * was active. Once the guest is running, used_length can change. Used >>> to >>> + * register/unregister uffd handlers and as the size of the recv >>> bitmap. >>> + * Receiving any page beyond this length will bail out, as it could >>> not have >>> + * been valid on the source. >>> + */ >>> + ram_addr_t postcopy_length; >>> }; >>> #endif >>> #endif >>> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c >>> index a36402722b..c68caf4e42 100644 >>> --- a/migration/postcopy-ram.c >>> +++ b/migration/postcopy-ram.c >>> @@ -17,6 +17,7 @@ >>> */ >>> >>> #include "qemu/osdep.h" >>> +#include "qemu/rcu.h" >>> #include "exec/target_page.h" >>> #include "migration.h" >>> #include "qemu-file.h" >>> @@ -31,6 +32,7 @@ >>> #include "qemu/error-report.h" >>> #include "trace.h" >>> #include "hw/boards.h" >>> +#include "exec/ramblock.h" >>> >>> /* Arbitrary limit on size of each discard command, >>> * keeps them around ~200 bytes >>> @@ -456,6 +458,13 @@ static int init_range(RAMBlock *rb, void *opaque) >>> ram_addr_t length = qemu_ram_get_used_length(rb); >>> trace_postcopy_init_range(block_name, host_addr, offset, length); >>> >>> + /* >>> + * Save the used_length before running the guest. In case we have to >>> + * resize RAM blocks when syncing RAM block sizes from the source >>> during >>> + * precopy, we'll update it manually via the ram block notifier. >>> + */ >>> + rb->postcopy_length = length; >>> + >>> /* >>> * We need the whole of RAM to be truly empty for postcopy, so things >>> * like ROMs and any data tables built during init must be zero'd >>> @@ -478,7 +487,7 @@ static int cleanup_range(RAMBlock *rb, void *opaque) >>> const char *block_name = qemu_ram_get_idstr(rb); >>> void *host_addr = qemu_ram_get_host_addr(rb); >>> ram_addr_t offset = qemu_ram_get_offset(rb); >>> - ram_addr_t length = qemu_ram_get_used_length(rb); >>> + ram_addr_t length = rb->postcopy_length; >>> MigrationIncomingState *mis = opaque; >>> struct uffdio_range range_struct; >>> trace_postcopy_cleanup_range(block_name, host_addr, offset, length); >>> @@ -600,7 +609,7 @@ static int nhp_range(RAMBlock *rb, void *opaque) >>> const char *block_name = qemu_ram_get_idstr(rb); >>> void *host_addr = qemu_ram_get_host_addr(rb); >>> ram_addr_t offset = qemu_ram_get_offset(rb); >>> - ram_addr_t length = qemu_ram_get_used_length(rb); >>> + ram_addr_t length = rb->postcopy_length; >>> trace_postcopy_nhp_range(block_name, host_addr, offset, length); >>> >>> /* >>> @@ -644,7 +653,7 @@ static int ram_block_enable_notify(RAMBlock *rb, void >>> *opaque) >>> struct uffdio_register reg_struct; >>> >>> reg_struct.range.start = (uintptr_t)qemu_ram_get_host_addr(rb); >>> - reg_struct.range.len = qemu_ram_get_used_length(rb); >>> + reg_struct.range.len = rb->postcopy_length; >>> reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING; >>> >>> /* Now tell our userfault_fd that it's responsible for this area */ >>> diff --git a/migration/ram.c b/migration/ram.c >>> index ab1f5534cf..6d1dcb362c 100644 >>> --- a/migration/ram.c >>> +++ b/migration/ram.c >>> @@ -244,7 +244,7 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file, >>> return -1; >>> } >>> >>> - nbits = block->used_length >> TARGET_PAGE_BITS; >>> + nbits = block->postcopy_length >> TARGET_PAGE_BITS; >>> >>> /* >>> * Make sure the tmp bitmap buffer is big enough, e.g., on 32bit >>> @@ -3159,7 +3159,13 @@ static int ram_load_postcopy(QEMUFile *f) >>> break; >>> } >>> >>> - if (!offset_in_ramblock(block, addr)) { >>> + /* >>> + * Relying on used_length is racy and can result in false >>> positives. >>> + * We might place pages beyond used_length in case RAM was >>> shrunk >>> + * while in postcopy, which is fine - trying to place via >>> + * UFFDIO_COPY/UFFDIO_ZEROPAGE will never segfault. >>> + */ >>> + if (!block->host || addr >= block->postcopy_length) { >>> error_report("Illegal RAM offset " RAM_ADDR_FMT, addr); >>> ret = -EINVAL; >>> break; >>> @@ -3744,6 +3750,7 @@ static void >>> ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host, >>> rb->idstr); >>> } >>> } >>> + rb->postcopy_length = new_size; >> >> With this change, postcopy_length will be the same as used_length? >> >> I thought you wanted to cache that value when starting the postcopy >> phase so postcopy_length should be constant after set once. Did I >> misunderstood? > > So, my understanding on the migration target: > > 1. Source VM started and initialized.
"Destination VM", sorry. -- Thanks, David / dhildenb