Re: [PATCH v3 10/13] migration/ram: Handle RAM block resizes during postcopy

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Hildenbrand (david@redhat.com) wrote:
> On 06.03.20 17:56, Dr. David Alan Gilbert wrote:
> > * David Hildenbrand (david@redhat.com) 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.
> >>
> >> Reviewed-by: Peter Xu <peterx@redhat.com>
> >> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >> Cc: Juan Quintela <quintela@redhat.com>
> >> Cc: Eduardo Habkost <ehabkost@redhat.com>
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: Igor Mammedov <imammedo@redhat.com>
> >> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> >> Cc: Richard Henderson <richard.henderson@linaro.org>
> >> Cc: Shannon Zhao <shannon.zhao@linaro.org>
> >> Cc: Alex Bennée <alex.bennee@linaro.org>
> >> Cc: Peter Xu <peterx@redhat.com>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >> ---
> >>  include/exec/ramblock.h  | 10 ++++++++++
> >>  migration/postcopy-ram.c | 15 ++++++++++++---
> >>  migration/ram.c          | 11 +++++++++--
> >>  3 files changed, 31 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
> >> index 07d50864d8..664701b759 100644
> >> --- a/include/exec/ramblock.h
> >> +++ b/include/exec/ramblock.h
> >> @@ -59,6 +59,16 @@ struct RAMBlock {
> >>       */
> >>      unsigned long *clear_bmap;
> >>      uint8_t clear_bmap_shift;
> >> +
> >> +    /*
> >> +     * RAM block length that corresponds to the used_length on the migration
> >> +     * source (after RAM block sizes were synchronized). Especially, after
> >> +     * starting to run the guest, used_length and postcopy_length can differ.
> >> +     * Used to register/unregister uffd handlers and as the size of the received
> >> +     * 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 1a5ff07997..ee5c3d5784 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
> >> @@ -3160,7 +3160,13 @@ 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.
> >> +             */
> > 
> > Is this actually safe? Imagine that the region had got shrunk, would it
> > still be mmap'd in there - or could there now be a space where something
> > else might have landed in?
> 
> Yes, it's safe. The mapping of resizeable RAM blocks will currently not
> change when resized. See patch #13 on how this is handled when the
> mapping actually change (preparation for resizeable allocations [1]).

OK, in that case,


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> [1]
> https://lore.kernel.org/qemu-devel/20200305142945.216465-1-david@redhat.com/
> 
> -- 
> Thanks,
> 
> David / dhildenb
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK