Re: [PATCH v1 10/13] migrate/ram: Handle RAM block resizes during postcopy

[David Hildenbrand <david@redhat.com>]

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" <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  |  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