Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability

[Peter Xu]

On Thu, Mar 30, 2023 at 03:03:20PM -0300, Fabiano Rosas wrote:
> From: Nikolay Borisov <nborisov@suse.com>
> 
> Implement 'fixed-ram' feature. The core of the feature is to ensure that
> each ram page of the migration stream has a specific offset in the
> resulting migration stream. The reason why we'd want such behavior are
> two fold:
> 
>  - When doing a 'fixed-ram' migration the resulting file will have a
>    bounded size, since pages which are dirtied multiple times will
>    always go to a fixed location in the file, rather than constantly
>    being added to a sequential stream. This eliminates cases where a vm
>    with, say, 1G of ram can result in a migration file that's 10s of
>    GBs, provided that the workload constantly redirties memory.
> 
>  - It paves the way to implement DIO-enabled save/restore of the
>    migration stream as the pages are ensured to be written at aligned
>    offsets.
> 
> The feature requires changing the stream format. First, a bitmap is
> introduced which tracks which pages have been written (i.e are
> dirtied) during migration and subsequently it's being written in the
> resulting file, again at a fixed location for every ramblock. Zero
> pages are ignored as they'd be zero in the destination migration as
> well. With the changed format data would look like the following:
> 
> |name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages|

What happens with huge pages?  Would page size matter here?

I would assume it's fine it uses a constant (small) page size, assuming
that should match with the granule that qemu tracks dirty (which IIUC is
the host page size not guest's).

But I didn't yet pay any further thoughts on that, maybe it would be
worthwhile in all cases to record page sizes here to be explicit or the
meaning of bitmap may not be clear (and then the bitmap_size will be a
field just for sanity check too).

If postcopy might be an option, we'd want the page size to be the host page
size because then looking up the bitmap will be straightforward, deciding
whether we should copy over page (UFFDIO_COPY) or fill in with zeros
(UFFDIO_ZEROPAGE).

> 
> * pc - refers to the page_size/mr->addr members, so newly added members
> begin from "bitmap_size".

Could you elaborate more on what's the pc?

I also didn't see this *pc in below migration.rst update.

> 
> This layout is initialized during ram_save_setup so instead of having a
> sequential stream of pages that follow the ramblock headers the dirty
> pages for a ramblock follow its header. Since all pages have a fixed
> location RAM_SAVE_FLAG_EOS is no longer generated on every migration
> iteration but there is effectively a single RAM_SAVE_FLAG_EOS right at
> the end.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  docs/devel/migration.rst | 36 +++++++++++++++
>  include/exec/ramblock.h  |  8 ++++
>  migration/migration.c    | 51 +++++++++++++++++++++-
>  migration/migration.h    |  1 +
>  migration/ram.c          | 94 +++++++++++++++++++++++++++++++++-------
>  migration/savevm.c       |  1 +
>  qapi/migration.json      |  2 +-
>  7 files changed, 176 insertions(+), 17 deletions(-)
> 
> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
> index 1080211f8e..84112d7f3f 100644
> --- a/docs/devel/migration.rst
> +++ b/docs/devel/migration.rst
> @@ -568,6 +568,42 @@ Others (especially either older devices or system 
> devices which for
>  some reason don't have a bus concept) make use of the ``instance id``
>  for otherwise identically named devices.
>  
> +Fixed-ram format
> +----------------
> +
> +When the ``fixed-ram`` capability is enabled, a slightly different
> +stream format is used for the RAM section. Instead of having a
> +sequential stream of pages that follow the RAMBlock headers, the dirty
> +pages for a RAMBlock follow its header. This ensures that each RAM
> +page has a fixed offset in the resulting migration stream.
> +
> +  - RAMBlock 1
> +
> +    - ID string length
> +    - ID string
> +    - Used size
> +    - Shadow bitmap size
> +    - Pages offset in migration stream*
> +
> +  - Shadow bitmap
> +  - Sequence of pages for RAMBlock 1 (* offset points here)
> +
> +  - RAMBlock 2
> +
> +    - ID string length
> +    - ID string
> +    - Used size
> +    - Shadow bitmap size
> +    - Pages offset in migration stream*
> +
> +  - Shadow bitmap
> +  - Sequence of pages for RAMBlock 2 (* offset points here)
> +
> +The ``fixed-ram`` capaility can be enabled in both source and
> +destination with:
> +
> +    ``migrate_set_capability fixed-ram on``
> +
>  Return path
>  -----------
>  
> diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
> index adc03df59c..4360c772c2 100644
> --- a/include/exec/ramblock.h
> +++ b/include/exec/ramblock.h
> @@ -43,6 +43,14 @@ struct RAMBlock {
>      size_t page_size;
>      /* dirty bitmap used during migration */
>      unsigned long *bmap;
> +    /* shadow dirty bitmap used when migrating to a file */
> +    unsigned long *shadow_bmap;
> +    /*
> +     * offset in the file pages belonging to this ramblock are saved,
> +     * used only during migration to a file.
> +     */
> +    off_t bitmap_offset;
> +    uint64_t pages_offset;
>      /* bitmap of already received pages in postcopy */
>      unsigned long *receivedmap;
>  
> diff --git a/migration/migration.c b/migration/migration.c
> index 177fb0de0f..29630523e2 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -168,7 +168,8 @@ 
> INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
>      MIGRATION_CAPABILITY_XBZRLE,
>      MIGRATION_CAPABILITY_X_COLO,
>      MIGRATION_CAPABILITY_VALIDATE_UUID,
> -    MIGRATION_CAPABILITY_ZERO_COPY_SEND);
> +    MIGRATION_CAPABILITY_ZERO_COPY_SEND,
> +    MIGRATION_CAPABILITY_FIXED_RAM);
>  
>  /* When we add fault tolerance, we could have several
>     migrations at once.  For now we don't need to add
> @@ -1341,6 +1342,28 @@ static bool migrate_caps_check(bool *cap_list,
>      }
>  #endif
>  
> +    if (cap_list[MIGRATION_CAPABILITY_FIXED_RAM]) {
> +        if (cap_list[MIGRATION_CAPABILITY_MULTIFD]) {
> +            error_setg(errp, "Directly mapped memory incompatible with 
> multifd");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_XBZRLE]) {
> +            error_setg(errp, "Directly mapped memory incompatible with 
> xbzrle");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
> +            error_setg(errp, "Directly mapped memory incompatible with 
> compression");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
> +            error_setg(errp, "Directly mapped memory incompatible with 
> postcopy ram");
> +            return false;
> +        }
> +    }
> +
>      if (cap_list[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
>          /* This check is reasonably expensive, so only when it's being
>           * set the first time, also it's only the destination that needs
> @@ -2736,6 +2759,11 @@ MultiFDCompression migrate_multifd_compression(void)
>      return s->parameters.multifd_compression;
>  }
>  
> +int migrate_fixed_ram(void)
> +{
> +    return 
> migrate_get_current()->enabled_capabilities[MIGRATION_CAPABILITY_FIXED_RAM];
> +}
> +
>  int migrate_multifd_zlib_level(void)
>  {
>      MigrationState *s;
> @@ -4324,6 +4352,20 @@ fail:
>      return NULL;
>  }
>  
> +static int migrate_check_fixed_ram(MigrationState *s, Error **errp)
> +{
> +    if (!s->enabled_capabilities[MIGRATION_CAPABILITY_FIXED_RAM]) {
> +        return 0;
> +    }
> +
> +    if (!qemu_file_is_seekable(s->to_dst_file)) {
> +        error_setg(errp, "Directly mapped memory requires a seekable 
> transport");
> +        return -1;
> +    }
> +
> +    return 0;
> +}
> +
>  void migrate_fd_connect(MigrationState *s, Error *error_in)
>  {
>      Error *local_err = NULL;
> @@ -4390,6 +4432,12 @@ void migrate_fd_connect(MigrationState *s, Error 
> *error_in)
>          }
>      }
>  
> +    if (migrate_check_fixed_ram(s, &local_err) < 0) {

This check might be too late afaict, QMP cmd "migrate" could have already
succeeded.

Can we do an early check in / close to qmp_migrate()?  The idea is we fail
at the QMP migrate command there.

> +        migrate_fd_cleanup(s);
> +        migrate_fd_error(s, local_err);
> +        return;
> +    }
> +
>      if (resume) {
>          /* Wakeup the main migration thread to do the recovery */
>          migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_PAUSED,
> @@ -4519,6 +4567,7 @@ static Property migration_properties[] = {
>      DEFINE_PROP_STRING("tls-authz", MigrationState, parameters.tls_authz),
>  
>      /* Migration capabilities */
> +    DEFINE_PROP_MIG_CAP("x-fixed-ram", MIGRATION_CAPABILITY_FIXED_RAM),
>      DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
>      DEFINE_PROP_MIG_CAP("x-rdma-pin-all", MIGRATION_CAPABILITY_RDMA_PIN_ALL),
>      DEFINE_PROP_MIG_CAP("x-auto-converge", 
> MIGRATION_CAPABILITY_AUTO_CONVERGE),
> diff --git a/migration/migration.h b/migration/migration.h
> index 2da2f8a164..8cf3caecfe 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -416,6 +416,7 @@ bool migrate_zero_blocks(void);
>  bool migrate_dirty_bitmaps(void);
>  bool migrate_ignore_shared(void);
>  bool migrate_validate_uuid(void);
> +int migrate_fixed_ram(void);
>  
>  bool migrate_auto_converge(void);
>  bool migrate_use_multifd(void);
> diff --git a/migration/ram.c b/migration/ram.c
> index 96e8a19a58..56f0f782c8 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1310,9 +1310,14 @@ static int save_zero_page_to_file(PageSearchStatus 
> *pss,
>      int len = 0;
>  
>      if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {
> -        len += save_page_header(pss, block, offset | RAM_SAVE_FLAG_ZERO);
> -        qemu_put_byte(file, 0);
> -        len += 1;
> +        if (migrate_fixed_ram()) {
> +            /* for zero pages we don't need to do anything */
> +            len = 1;

I think you wanted to increase the "duplicated" counter, but this will also
increase ram-transferred even though only 1 byte.

Perhaps just pass a pointer to keep the bytes, and return true/false to
increase the counter (to make everything accurate)?

> +        } else {
> +            len += save_page_header(pss, block, offset | RAM_SAVE_FLAG_ZERO);
> +            qemu_put_byte(file, 0);
> +            len += 1;
> +        }
>          ram_release_page(block->idstr, offset);
>      }
>      return len;
> @@ -1394,14 +1399,20 @@ static int save_normal_page(PageSearchStatus *pss, 
> RAMBlock *block,
>  {
>      QEMUFile *file = pss->pss_channel;
>  
> -    ram_transferred_add(save_page_header(pss, block,
> -                                         offset | RAM_SAVE_FLAG_PAGE));
> -    if (async) {
> -        qemu_put_buffer_async(file, buf, TARGET_PAGE_SIZE,
> -                              migrate_release_ram() &&
> -                              migration_in_postcopy());
> +    if (migrate_fixed_ram()) {
> +        qemu_put_buffer_at(file, buf, TARGET_PAGE_SIZE,
> +                           block->pages_offset + offset);
> +        set_bit(offset >> TARGET_PAGE_BITS, block->shadow_bmap);
>      } else {
> -        qemu_put_buffer(file, buf, TARGET_PAGE_SIZE);
> +        ram_transferred_add(save_page_header(pss, block,
> +                                             offset | RAM_SAVE_FLAG_PAGE));
> +        if (async) {
> +            qemu_put_buffer_async(file, buf, TARGET_PAGE_SIZE,
> +                                  migrate_release_ram() &&
> +                                  migration_in_postcopy());
> +        } else {
> +            qemu_put_buffer(file, buf, TARGET_PAGE_SIZE);
> +        }
>      }
>      ram_transferred_add(TARGET_PAGE_SIZE);
>      stat64_add(&ram_atomic_counters.normal, 1);
> @@ -2731,6 +2742,8 @@ static void ram_save_cleanup(void *opaque)
>          block->clear_bmap = NULL;
>          g_free(block->bmap);
>          block->bmap = NULL;
> +        g_free(block->shadow_bmap);
> +        block->shadow_bmap = NULL;
>      }
>  
>      xbzrle_cleanup();
> @@ -3098,6 +3111,7 @@ static void ram_list_init_bitmaps(void)
>               */
>              block->bmap = bitmap_new(pages);
>              bitmap_set(block->bmap, 0, pages);
> +            block->shadow_bmap = bitmap_new(block->used_length >> 
> TARGET_PAGE_BITS);
>              block->clear_bmap_shift = shift;
>              block->clear_bmap = bitmap_new(clear_bmap_size(pages, shift));
>          }
> @@ -3287,6 +3301,33 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>              if (migrate_ignore_shared()) {
>                  qemu_put_be64(f, block->mr->addr);
>              }
> +
> +            if (migrate_fixed_ram()) {
> +                long num_pages = block->used_length >> TARGET_PAGE_BITS;
> +                long bitmap_size = BITS_TO_LONGS(num_pages) * 
> sizeof(unsigned long);
> +
> +                /* Needed for external programs (think analyze-migration.py) 
> */
> +                qemu_put_be32(f, bitmap_size);
> +
> +                /*
> +                 * The bitmap starts after pages_offset, so add 8 to
> +                 * account for the pages_offset size.
> +                 */
> +                block->bitmap_offset = qemu_get_offset(f) + 8;
> +
> +                /*
> +                 * Make pages_offset aligned to 1 MiB to account for
> +                 * migration file movement between filesystems with
> +                 * possibly different alignment restrictions when
> +                 * using O_DIRECT.
> +                 */
> +                block->pages_offset = ROUND_UP(block->bitmap_offset +
> +                                               bitmap_size, 0x100000);
> +                qemu_put_be64(f, block->pages_offset);
> +
> +                /* Now prepare offset for next ramblock */
> +                qemu_set_offset(f, block->pages_offset + block->used_length, 
> SEEK_SET);
> +            }
>          }
>      }
>  
> @@ -3306,6 +3347,18 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>      return 0;
>  }
>  
> +static void ram_save_shadow_bmap(QEMUFile *f)
> +{
> +    RAMBlock *block;
> +
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
> +        long num_pages = block->used_length >> TARGET_PAGE_BITS;
> +        long bitmap_size = BITS_TO_LONGS(num_pages) * sizeof(unsigned long);
> +        qemu_put_buffer_at(f, (uint8_t *)block->shadow_bmap, bitmap_size,
> +                           block->bitmap_offset);
> +    }
> +}
> +
>  /**
>   * ram_save_iterate: iterative stage for migration
>   *
> @@ -3413,9 +3466,15 @@ out:
>              return ret;
>          }
>  
> -        qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
> -        qemu_fflush(f);
> -        ram_transferred_add(8);
> +        /*
> +         * For fixed ram we don't want to pollute the migration stream with
> +         * EOS flags.
> +         */
> +        if (!migrate_fixed_ram()) {
> +            qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
> +            qemu_fflush(f);
> +            ram_transferred_add(8);
> +        }
>  
>          ret = qemu_file_get_error(f);
>      }
> @@ -3461,6 +3520,9 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>              pages = ram_find_and_save_block(rs);
>              /* no more blocks to sent */
>              if (pages == 0) {
> +                if (migrate_fixed_ram()) {
> +                    ram_save_shadow_bmap(f);
> +                }
>                  break;
>              }
>              if (pages < 0) {
> @@ -3483,8 +3545,10 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>          return ret;
>      }
>  
> -    qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
> -    qemu_fflush(f);
> +    if (!migrate_fixed_ram()) {
> +        qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
> +        qemu_fflush(f);
> +    }
>  
>      return 0;
>  }
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 92102c1fe5..1f1bc19224 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -241,6 +241,7 @@ static bool should_validate_capability(int capability)
>      /* Validate only new capabilities to keep compatibility. */
>      switch (capability) {
>      case MIGRATION_CAPABILITY_X_IGNORE_SHARED:
> +    case MIGRATION_CAPABILITY_FIXED_RAM:
>          return true;
>      default:
>          return false;
> diff --git a/qapi/migration.json b/qapi/migration.json
> index c84fa10e86..22eea58ce3 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -485,7 +485,7 @@
>  ##
>  { 'enum': 'MigrationCapability',
>    'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> -           'compress', 'events', 'postcopy-ram',
> +           'compress', 'events', 'postcopy-ram', 'fixed-ram',
>             { 'name': 'x-colo', 'features': [ 'unstable' ] },
>             'release-ram',
>             'block', 'return-path', 'pause-before-switchover', 'multifd',
> -- 
> 2.35.3
> 

-- 
Peter Xu