Re: [RFC PATCH v3 08/30] migration/ram: Add outgoing 'fixed-ram' migration

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Mon, Nov 27, 2023 at 05:25:50PM -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):        With fixed-ram (new):
>> 
>>  ---------------------               --------------------------------
>>  | ramblock 1 header |               | ramblock 1 header            |
>>  ---------------------               --------------------------------
>>  | ramblock 2 header |               | ramblock 1 fixed-ram header  |
>>  ---------------------               --------------------------------
>>  | ...               |               | padding to next 1MB boundary |
>>  ---------------------               | ...                          |
>>  | ramblock n header |               --------------------------------
>>  ---------------------               | ramblock 1 pages             |
>>  | RAM_SAVE_FLAG_EOS |               | ...                          |
>>  ---------------------               --------------------------------
>>  | stream of pages   |               | ramblock 2 header            |
>>  | (iter 1)          |               --------------------------------
>>  | ...               |               | ramblock 2 fixed-ram header  |
>>  ---------------------               --------------------------------
>>  | RAM_SAVE_FLAG_EOS |               | padding to next 1MB boundary |
>>  ---------------------               | ...                          |
>>  | stream of pages   |               --------------------------------
>>  | (iter 2)          |               | 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>
>> ---
>> - used a macro for alignment value
>> - documented alignment assumptions
>> - moved shadow_bmap debug code to multifd patch
>> - did NOT use used_length for bmap, it breaks dirty page tracking somehow
>> - uncommented the capability enabling
>> - accounted for the bitmap size with ram_transferred_add()
>> ---
>>  include/exec/ramblock.h |   8 +++
>>  migration/ram.c         | 121 +++++++++++++++++++++++++++++++++++++---
>>  2 files changed, 120 insertions(+), 9 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;
>
> What is a "shadow dirty bitmap"?  It's pretty unclear to me.
>
> AFAICT it's actually a "page present" bitmap, while taking zero pages as
> "not present", no?
>

Yes, something like that. It's the bitmap of pages written to the
migration file.

>> +    /*
>> +     * 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;
>
> Let's have a section to put fixed-ram data?
>
>   /*
>    * Below fields are only used by fixed-ram migration.
>    */
>   ...
>
>>      /* bitmap of already received pages in postcopy */
>>      unsigned long *receivedmap;
>>  
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 8c7886ab79..4a0ab8105f 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -94,6 +94,18 @@
>>  #define RAM_SAVE_FLAG_MULTIFD_FLUSH    0x200
>>  /* We can't use any flag that is bigger than 0x200 */
>>  
>> +/*
>> + * fixed-ram migration supports O_DIRECT, so we need to make sure the
>> + * userspace buffer, the IO operation size and the file offset are
>> + * aligned according to the underlying device's block size. The first
>> + * two are already aligned to page size, but we need to add padding to
>> + * the file to align the offset.  We cannot read the block size
>> + * dynamically because the migration file can be moved between
>> + * different systems, so use 1M to cover most block sizes and to keep
>> + * the file offset aligned at page size as well.
>> + */
>> +#define FIXED_RAM_FILE_OFFSET_ALIGNMENT 0x100000
>> +
>>  XBZRLECacheStats xbzrle_counters;
>>  
>>  /* used by the search for pages to send */
>> @@ -1127,12 +1139,18 @@ 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);
>>  
>>      /*
>> @@ -1190,14 +1208,20 @@ 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);
>> @@ -2413,6 +2437,8 @@ 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();
>> @@ -2780,6 +2806,7 @@ static void ram_list_init_bitmaps(void)
>>               */
>>              block->bmap = bitmap_new(pages);
>>              bitmap_set(block->bmap, 0, pages);
>> +            block->shadow_bmap = bitmap_new(pages);
>
> We can avoid creating this bitmap if !fixed-ram.
>

ack

>>              block->clear_bmap_shift = shift;
>>              block->clear_bmap = bitmap_new(clear_bmap_size(pages, shift));
>>          }
>> @@ -2917,6 +2944,58 @@ void qemu_guest_free_page_hint(void *addr, size_t len)
>>      }
>>  }
>>  
>> +#define FIXED_RAM_HDR_VERSION 1
>> +struct FixedRamHeader {
>> +    uint32_t version;
>> +    /*
>> +     * The target's page size, so we know how many pages are in the
>> +     * bitmap.
>> +     */
>> +    uint64_t page_size;
>> +    /*
>> +     * The offset in the migration file where the pages bitmap is
>> +     * found.
>
> s/found/stored/?
>
>> +     */
>> +    uint64_t bitmap_offset;
>> +    /*
>> +     * The offset in the migration file where the actual pages (data)
>> +     * are found.
>
> same?
>
>> +     */
>> +    uint64_t pages_offset;
>> +    /* end of v1 */
>
> I think we can drop this.
>
>> +} QEMU_PACKED;
>> +typedef struct FixedRamHeader FixedRamHeader;
>> +
>> +static void fixed_ram_insert_header(QEMUFile *file, RAMBlock *block)
>> +{
>> +    g_autofree FixedRamHeader *header;
>
> Let's either inline the g_new0() or initialize it to NULL? Just in case.

Ouch, that's just wrong. I'll fix it.

>> +    size_t header_size, bitmap_size;
>> +    long num_pages;
>> +
>> +    header = g_new0(FixedRamHeader, 1);
>> +    header_size = sizeof(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,
>> +                                   FIXED_RAM_FILE_OFFSET_ALIGNMENT);
>> +
>> +    header->version = cpu_to_be32(FIXED_RAM_HDR_VERSION);
>> +    header->page_size = cpu_to_be64(TARGET_PAGE_SIZE);
>> +    header->bitmap_offset = cpu_to_be64(block->bitmap_offset);
>> +    header->pages_offset = cpu_to_be64(block->pages_offset);
>> +
>> +    qemu_put_buffer(file, (uint8_t *) header, header_size);
>> +}
>> +
>>  /*
>>   * Each of ram_save_setup, ram_save_iterate and ram_save_complete has
>>   * long-running RCU critical section.  When rcu-reclaims in the code
>> @@ -2966,6 +3045,13 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>>              if (migrate_ignore_shared()) {
>>                  qemu_put_be64(f, block->mr->addr);
>>              }
>> +
>> +            if (migrate_fixed_ram()) {
>> +                fixed_ram_insert_header(f, block);
>> +                /* prepare offset for next ramblock */
>> +                qemu_set_offset(f, block->pages_offset + block->used_length,
>> +                                SEEK_SET);
>
> How about moving this line into fixed_ram_insert_header()?  Perhaps also
> rename to fixed_ram_setup_ramblock()?
>

Ok.

>> +            }
>>          }
>>      }
>>  
>> @@ -2999,6 +3085,19 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>>      return qemu_fflush(f);
>>  }
>>  
>> +static void ram_save_shadow_bmap(QEMUFile *f)
>
> [may need a rename after we decide a better name for the bitmap; "shadow"
>  is probably not the one..]
>
>> +{
>> +    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);
>
> We may want to check for IO errors, either here, or (if too frequent) maybe
> once and for all right before the final completion of migration?  If the
> latter, we may want to keep a comment around here explaining on error conditions.
>

Ok.

>> +        ram_transferred_add(bitmap_size);
>> +    }
>> +}
>> +
>>  /**
>>   * ram_save_iterate: iterative stage for migration
>>   *
>> @@ -3188,6 +3287,10 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>>          return ret;
>>      }
>>  
>> +    if (migrate_fixed_ram()) {
>> +        ram_save_shadow_bmap(f);
>> +    }
>> +
>>      if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
>>          qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>>      }
>> -- 
>> 2.35.3
>>