Re: [PATCH v2 15/29] migration/ram: Add support for 'fixed-ram' outgoing migration

[Daniel P . Berrangé]

On Mon, Oct 23, 2023 at 05:35:54PM -0300, Fabiano Rosas wrote:
> From: Nikolay Borisov <nborisov@suse.com>
> 
> Implement the outgoing migration side for the 'fixed-ram' capability.
> 
> A bitmap is introduced to track which pages have been written in the
> migration file. Pages are written at a fixed location for every
> ramblock. Zero pages are ignored as they'd be zero in the destination
> migration as well.
> 
> The migration stream is altered to put the dirty pages for a ramblock
> after its header instead of having a sequential stream of pages that
> follow the ramblock headers. Since all pages have a fixed location,
> RAM_SAVE_FLAG_EOS is no longer generated on every migration iteration.
> 
> Without fixed-ram (current):
> 
> ramblock 1 header|ramblock 2 header|...|RAM_SAVE_FLAG_EOS|stream of
>  pages (iter 1)|RAM_SAVE_FLAG_EOS|stream of pages (iter 2)|...

Clearer to diagram this vertically I think

 ---------------------
 | ramblock 1 header |
 ---------------------
 | ramblock 2 header |
 ---------------------
 | ...               |
 ---------------------
 | ramblock n header |
 ---------------------
 | RAM_SAVE_FLAG_EOS |
 ---------------------
 | stream of pages   |
 | (iter 1)          |
 | ...               |
 ---------------------
 | RAM_SAVE_FLAG_EOS |
 ---------------------
 | stream of pages   |
 | (iter 2)          |
 | ...               |
 ---------------------
 | ...               |
 ---------------------
 

> 
> With fixed-ram (new):
> 
> ramblock 1 header|ramblock 1 fixed-ram header|ramblock 1 pages (fixed
>  offsets)|ramblock 2 header|ramblock 2 fixed-ram header|ramblock 2
>  pages (fixed offsets)|...|RAM_SAVE_FLAG_EOS

If I'm reading the code correctly the new format has some padding
such that each "ramblock pages" region starts on a 1 MB boundary.

eg so we get:

 --------------------------------
 | ramblock 1 header            |
 --------------------------------
 | ramblock 1 fixed-ram header  |
 --------------------------------
 | padding to next 1MB boundary |
 | ...                          |
 --------------------------------
 | ramblock 1 pages             |
 | ...                          |
 --------------------------------
 | ramblock 2 header            |
 --------------------------------
 | ramblock 2 fixed-ram header  |
 --------------------------------
 | padding to next 1MB boundary |
 | ...                          |
 --------------------------------
 | ramblock 2 pages             |
 | ...                          |
 --------------------------------
 | ...                          |
 --------------------------------
 | RAM_SAVE_FLAG_EOS            |
 --------------------------------
 | ...                          |
 -------------------------------

> 
> where:
>  - ramblock header: the generic information for a ramblock, such as
>    idstr, used_len, etc.
> 
>  - ramblock fixed-ram header: the new information added by this
>    feature: bitmap of pages written, bitmap size and offset of pages
>    in the migration file.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  include/exec/ramblock.h |  8 ++++
>  migration/options.c     |  3 --
>  migration/ram.c         | 98 ++++++++++++++++++++++++++++++++++++-----
>  3 files changed, 96 insertions(+), 13 deletions(-)
> 
> diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
> index 69c6a53902..e0e3f16852 100644
> --- a/include/exec/ramblock.h
> +++ b/include/exec/ramblock.h
> @@ -44,6 +44,14 @@ 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/options.c b/migration/options.c
> index 2622d8c483..9f693d909f 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -271,12 +271,9 @@ bool migrate_events(void)
>  
>  bool migrate_fixed_ram(void)
>  {
> -/*
>      MigrationState *s = migrate_get_current();
>  
>      return s->capabilities[MIGRATION_CAPABILITY_FIXED_RAM];
> -*/
> -    return false;
>  }
>  
>  bool migrate_ignore_shared(void)
> diff --git a/migration/ram.c b/migration/ram.c
> index 92769902bb..152a03604f 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1157,12 +1157,18 @@ static int save_zero_page(RAMState *rs, 
> PageSearchStatus *pss,
>          return 0;
>      }
>  
> +    stat64_add(&mig_stats.zero_pages, 1);
> +
> +    if (migrate_fixed_ram()) {
> +        /* zero pages are not transferred with fixed-ram */
> +        clear_bit(offset >> TARGET_PAGE_BITS, pss->block->shadow_bmap);
> +        return 1;
> +    }
> +
>      len += save_page_header(pss, file, pss->block, offset | 
> RAM_SAVE_FLAG_ZERO);
>      qemu_put_byte(file, 0);
>      len += 1;
>      ram_release_page(pss->block->idstr, offset);
> -
> -    stat64_add(&mig_stats.zero_pages, 1);
>      ram_transferred_add(len);
>  
>      /*
> @@ -1220,14 +1226,20 @@ static int save_normal_page(PageSearchStatus *pss, 
> RAMBlock *block,
>  {
>      QEMUFile *file = pss->pss_channel;
>  
> -    ram_transferred_add(save_page_header(pss, pss->pss_channel, 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, pss->pss_channel, 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(&mig_stats.normal_pages, 1);
> @@ -2475,6 +2487,8 @@ 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();
> @@ -2842,6 +2856,7 @@ 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));
>          }
> @@ -2979,6 +2994,44 @@ void qemu_guest_free_page_hint(void *addr, size_t len)
>      }
>  }
>  
> +#define FIXED_RAM_HDR_VERSION 1
> +struct FixedRamHeader {
> +    uint32_t version;
> +    uint64_t page_size;
> +    uint64_t bitmap_offset;
> +    uint64_t pages_offset;
> +    /* end of v1 */
> +} QEMU_PACKED;
> +
> +static void fixed_ram_insert_header(QEMUFile *file, RAMBlock *block)
> +{
> +    g_autofree struct FixedRamHeader *header;
> +    size_t header_size, bitmap_size;
> +    long num_pages;
> +
> +    header = g_new0(struct FixedRamHeader, 1);
> +    header_size = sizeof(struct FixedRamHeader);
> +
> +    num_pages = block->used_length >> TARGET_PAGE_BITS;
> +    bitmap_size = BITS_TO_LONGS(num_pages) * sizeof(unsigned long);
> +
> +    /*
> +     * Save the file offsets of where the bitmap and the pages should
> +     * go as they are written at the end of migration and during the
> +     * iterative phase, respectively.
> +     */
> +    block->bitmap_offset = qemu_get_offset(file) + header_size;
> +    block->pages_offset = ROUND_UP(block->bitmap_offset +
> +                                   bitmap_size, 0x100000);

The 0x100000 gives us our 1 MB alignment.

This is quite an important thing, so can we put a nice clear
constant at the top of the file:

  /* Align fixed-ram pages data to the next 1 MB boundary */
  #define FIXED_RAM_PAGE_OFFSET_ALIGNMENT 0x100000

> @@ -3179,7 +3252,6 @@ out:
>          qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>          qemu_fflush(f);
>          ram_transferred_add(8);
> -
>          ret = qemu_file_get_error(f);
>      }
>      if (ret < 0) {

Supurious whitspace change, possibly better in whatever patch
introduced it, or dropped ?


With regards,
Daniel
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