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? Thanks, > return; > } > > -- > 2.24.1 > -- Peter Xu