Peter Xu <pet...@redhat.com> writes:
> On Mon, Oct 23, 2023 at 05:35:54PM -0300, Fabiano Rosas wrote:
>> From: Nikolay Borisov <nbori...@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)|...
>>
>> 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
>>
>> 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 <nbori...@suse.com>
>> Signed-off-by: Fabiano Rosas <faro...@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);
>
> Here we keep zero page accounting, but..
>
>> +
>> + 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));
>
> .. here we ignored normal page accounting.
>
> I think we should have the same behavior on both, perhaps keep them always?
>
This is the accounting for the header only if I'm not mistaken.
>> + 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);
Here's the page accounting.
>> @@ -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);
>
> AFAICT bmap should also use used_length. How about adding one more patch
> to change that, then you can use "pages" here?
It uses max_length. I don't know what are the effects of that
change. I'll look into it.
> See ram_mig_ram_block_resized() where we call migration_cancel() if resized.
>
>> 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);
>> +
>> + header->version = cpu_to_be32(FIXED_RAM_HDR_VERSION);
>> + header->page_size = cpu_to_be64(TARGET_PAGE_SIZE);
>
> This is the "page size" for the shadow bitmap, right? Shall we state it
> somewhere (e.g. explaining why it's not block->page_size)?
Ok.
> It's unfortunate that we already have things like:
>
> if (migrate_postcopy_ram() && block->page_size !=
> qemu_host_page_size) {
> qemu_put_be64(f, block->page_size);
> }
>
> But indeed we can't merge them because they seem to service different
> purpose.
>
>> + 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
>> @@ -3028,6 +3081,12 @@ 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);
>> + }
>> }
>> }
>>
>> @@ -3061,6 +3120,20 @@ 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);
>> + /* to catch any thread late sending pages */
>> + block->shadow_bmap = NULL;
>
> What is this for? Wouldn't this leak the buffer already?
>
Ah this is debug code. It's because of multifd. In this series I don't
use sem_sync because there's no packets. But doing so causes
multifd_send_sync_main() to return before the multifd channel has sent
all its pages. This is here so the channel crashes when writing the
bitmap.
I think it's worth it to keep it but I'd have to move it to the multifd
patch and free the bitmap properly.
Thanks!
