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. - Theoretically we could have resizes here already in the future (e.g., virtio-mem). Right now, not. 2. Precopy data is loaded - RAM block sizes will be synchronized to the source. 3. Guest starts running - Any RAM block resize will differ to the source. For postcopy, we have to "freeze" the RAM block size after 2, but before 3. So rb->postcopy_length will always correspond to used_length on the migration source. Interestingly, userfaultfd handlers etc. are registered just before 3, before any "resizes of interest" were handled. So, postcopy_length will match used_length until we start our guest and do resizes (shrink/grow) that are out of sync with our source. Makes sense? -- Thanks, David / dhildenb