* Peter Xu (pet...@redhat.com) wrote:
> Postcopy handles huge pages in a special way that currently we can only have
> one "channel" to transfer the page.
>
> It's because when we install pages using UFFDIO_COPY, we need to have the
> whole
> huge page ready, it also means we need to have a temp huge page when trying to
> receive the whole content of the page.
>
> Currently all maintainance around this tmp page is global: firstly we'll
> allocate a temp huge page, then we maintain its status mostly within
> ram_load_postcopy().
>
> To enable multiple channels for postcopy, the first thing we need to do is to
> prepare N temp huge pages as caching, one for each channel.
>
> Meanwhile we need to maintain the tmp huge page status per-channel too.
>
> To give some example, some local variables maintained in ram_load_postcopy()
> are listed; they are responsible for maintaining temp huge page status:
>
> - all_zero: this keeps whether this huge page contains all zeros
> - target_pages: this counts how many target pages have been copied
> - host_page: this keeps the host ptr for the page to install
>
> Move all these fields to be together with the temp huge pages to form a new
> structure called PostcopyTmpPage. Then for each (future) postcopy channel, we
> need one structure to keep the state around.
>
> For vanilla postcopy, obviously there's only one channel. It contains both
> precopy and postcopy pages.
>
> This patch teaches the dest migration node to start realize the possible
> number
> of postcopy channels by introducing the "postcopy_channels" variable. Its
> value is calculated when setup postcopy on dest node (during POSTCOPY_LISTEN
> phase).
>
> Vanilla postcopy will have channels=1, but when postcopy-preempt capability is
> enabled (in the future), we will boost it to 2 because even during partial
> sending of a precopy huge page we still want to preempt it and start sending
> the postcopy requested page right away (so we start to keep two temp huge
> pages; more if we want to enable multifd). In this patch there's a TODO
> marked
> for that; so far the channels is always set to 1.
>
> We need to send one "host huge page" on one channel only and we cannot split
> them, because otherwise the data upon the same huge page can locate on more
> than one channel so we need more complicated logic to manage. One temp host
> huge page for each channel will be enough for us for now.
>
> Postcopy will still always use the index=0 huge page even after this patch.
> However it prepares for the latter patches where it can start to use multiple
> channels (which needs src intervention, because only src knows which channel
> we
> should use).
>
> Signed-off-by: Peter Xu <pet...@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> ---
> migration/migration.h | 36 +++++++++++++++++++++++-
> migration/postcopy-ram.c | 60 ++++++++++++++++++++++++++++++----------
> migration/ram.c | 43 ++++++++++++++--------------
> migration/savevm.c | 12 ++++++++
> 4 files changed, 113 insertions(+), 38 deletions(-)
>
> diff --git a/migration/migration.h b/migration/migration.h
> index 8130b703eb..42c7395094 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -45,6 +45,24 @@ struct PostcopyBlocktimeContext;
> */
> #define CLEAR_BITMAP_SHIFT_MAX 31
>
> +/* This is an abstraction of a "temp huge page" for postcopy's purpose */
> +typedef struct {
> + /*
> + * This points to a temporary huge page as a buffer for UFFDIO_COPY.
> It's
> + * mmap()ed and needs to be freed when cleanup.
> + */
> + void *tmp_huge_page;
> + /*
> + * This points to the host page we're going to install for this temp
> page.
> + * It tells us after we've received the whole page, where we should put
> it.
> + */
> + void *host_addr;
> + /* Number of small pages copied (in size of TARGET_PAGE_SIZE) */
> + unsigned int target_pages;
> + /* Whether this page contains all zeros */
> + bool all_zero;
> +} PostcopyTmpPage;
> +
> /* State for the incoming migration */
> struct MigrationIncomingState {
> QEMUFile *from_src_file;
> @@ -81,7 +99,22 @@ struct MigrationIncomingState {
> QemuMutex rp_mutex; /* We send replies from multiple threads */
> /* RAMBlock of last request sent to source */
> RAMBlock *last_rb;
> - void *postcopy_tmp_page;
> + /*
> + * Number of postcopy channels including the default precopy channel, so
> + * vanilla postcopy will only contain one channel which contain both
> + * precopy and postcopy streams.
> + *
> + * This is calculated when the src requests to enable postcopy but before
> + * it starts. Its value can depend on e.g. whether postcopy preemption
> is
> + * enabled.
> + */
> + unsigned int postcopy_channels;
> + /*
> + * An array of temp host huge pages to be used, one for each postcopy
> + * channel.
> + */
> + PostcopyTmpPage *postcopy_tmp_pages;
> + /* This is shared for all postcopy channels */
> void *postcopy_tmp_zero_page;
> /* PostCopyFD's for external userfaultfds & handlers of shared memory */
> GArray *postcopy_remote_fds;
> @@ -391,5 +424,6 @@ bool migration_rate_limit(void);
> void migration_cancel(const Error *error);
>
> void populate_vfio_info(MigrationInfo *info);
> +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page);
>
> #endif
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index e662dd05cc..315f784965 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -525,9 +525,18 @@ int postcopy_ram_incoming_init(MigrationIncomingState
> *mis)
>
> static void postcopy_temp_pages_cleanup(MigrationIncomingState *mis)
> {
> - if (mis->postcopy_tmp_page) {
> - munmap(mis->postcopy_tmp_page, mis->largest_page_size);
> - mis->postcopy_tmp_page = NULL;
> + int i;
> +
> + if (mis->postcopy_tmp_pages) {
> + for (i = 0; i < mis->postcopy_channels; i++) {
> + if (mis->postcopy_tmp_pages[i].tmp_huge_page) {
> + munmap(mis->postcopy_tmp_pages[i].tmp_huge_page,
> + mis->largest_page_size);
> + mis->postcopy_tmp_pages[i].tmp_huge_page = NULL;
> + }
> + }
> + g_free(mis->postcopy_tmp_pages);
> + mis->postcopy_tmp_pages = NULL;
> }
>
> if (mis->postcopy_tmp_zero_page) {
> @@ -1091,17 +1100,30 @@ retry:
>
> static int postcopy_temp_pages_setup(MigrationIncomingState *mis)
> {
> - int err;
> -
> - mis->postcopy_tmp_page = mmap(NULL, mis->largest_page_size,
> - PROT_READ | PROT_WRITE,
> - MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> - if (mis->postcopy_tmp_page == MAP_FAILED) {
> - err = errno;
> - mis->postcopy_tmp_page = NULL;
> - error_report("%s: Failed to map postcopy_tmp_page %s",
> - __func__, strerror(err));
> - return -err;
> + PostcopyTmpPage *tmp_page;
> + int err, i, channels;
> + void *temp_page;
> +
> + /* TODO: will be boosted when enable postcopy preemption */
> + mis->postcopy_channels = 1;
> +
> + channels = mis->postcopy_channels;
> + mis->postcopy_tmp_pages = g_malloc0_n(sizeof(PostcopyTmpPage), channels);
> +
> + for (i = 0; i < channels; i++) {
> + tmp_page = &mis->postcopy_tmp_pages[i];
> + temp_page = mmap(NULL, mis->largest_page_size, PROT_READ |
> PROT_WRITE,
> + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> + if (temp_page == MAP_FAILED) {
> + err = errno;
> + error_report("%s: Failed to map postcopy_tmp_pages[%d]: %s",
> + __func__, i, strerror(err));
> + /* Clean up will be done later */
> + return -err;
> + }
> + tmp_page->tmp_huge_page = temp_page;
> + /* Initialize default states for each tmp page */
> + postcopy_temp_page_reset(tmp_page);
> }
>
> /*
> @@ -1351,6 +1373,16 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd,
> #endif
>
> /* -------------------------------------------------------------------------
> */
> +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page)
> +{
> + tmp_page->target_pages = 0;
> + tmp_page->host_addr = NULL;
> + /*
> + * This is set to true when reset, and cleared as long as we received any
> + * of the non-zero small page within this huge page.
> + */
> + tmp_page->all_zero = true;
> +}
>
> void postcopy_fault_thread_notify(MigrationIncomingState *mis)
> {
> diff --git a/migration/ram.c b/migration/ram.c
> index 91ca743ac8..36b0a53afe 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3640,11 +3640,8 @@ static int ram_load_postcopy(QEMUFile *f)
> bool place_needed = false;
> bool matches_target_page_size = false;
> MigrationIncomingState *mis = migration_incoming_get_current();
> - /* Temporary page that is later 'placed' */
> - void *postcopy_host_page = mis->postcopy_tmp_page;
> - void *host_page = NULL;
> - bool all_zero = true;
> - int target_pages = 0;
> + /* Currently we only use channel 0. TODO: use all the channels */
> + PostcopyTmpPage *tmp_page = &mis->postcopy_tmp_pages[0];
>
> while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
> ram_addr_t addr;
> @@ -3688,7 +3685,7 @@ static int ram_load_postcopy(QEMUFile *f)
> ret = -EINVAL;
> break;
> }
> - target_pages++;
> + tmp_page->target_pages++;
> matches_target_page_size = block->page_size == TARGET_PAGE_SIZE;
> /*
> * Postcopy requires that we place whole host pages atomically;
> @@ -3700,15 +3697,16 @@ static int ram_load_postcopy(QEMUFile *f)
> * however the source ensures it always sends all the components
> * of a host page in one chunk.
> */
> - page_buffer = postcopy_host_page +
> + page_buffer = tmp_page->tmp_huge_page +
> host_page_offset_from_ram_block_offset(block,
> addr);
> /* If all TP are zero then we can optimise the place */
> - if (target_pages == 1) {
> - host_page = host_page_from_ram_block_offset(block, addr);
> - } else if (host_page != host_page_from_ram_block_offset(block,
> - addr)) {
> + if (tmp_page->target_pages == 1) {
> + tmp_page->host_addr =
> + host_page_from_ram_block_offset(block, addr);
> + } else if (tmp_page->host_addr !=
> + host_page_from_ram_block_offset(block, addr)) {
> /* not the 1st TP within the HP */
> - error_report("Non-same host page %p/%p", host_page,
> + error_report("Non-same host page %p/%p", tmp_page->host_addr,
> host_page_from_ram_block_offset(block, addr));
> ret = -EINVAL;
> break;
> @@ -3718,10 +3716,11 @@ static int ram_load_postcopy(QEMUFile *f)
> * If it's the last part of a host page then we place the host
> * page
> */
> - if (target_pages == (block->page_size / TARGET_PAGE_SIZE)) {
> + if (tmp_page->target_pages ==
> + (block->page_size / TARGET_PAGE_SIZE)) {
> place_needed = true;
> }
> - place_source = postcopy_host_page;
> + place_source = tmp_page->tmp_huge_page;
> }
>
> switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
> @@ -3735,12 +3734,12 @@ static int ram_load_postcopy(QEMUFile *f)
> memset(page_buffer, ch, TARGET_PAGE_SIZE);
> }
> if (ch) {
> - all_zero = false;
> + tmp_page->all_zero = false;
> }
> break;
>
> case RAM_SAVE_FLAG_PAGE:
> - all_zero = false;
> + tmp_page->all_zero = false;
> if (!matches_target_page_size) {
> /* For huge pages, we always use temporary buffer */
> qemu_get_buffer(f, page_buffer, TARGET_PAGE_SIZE);
> @@ -3758,7 +3757,7 @@ static int ram_load_postcopy(QEMUFile *f)
> }
> break;
> case RAM_SAVE_FLAG_COMPRESS_PAGE:
> - all_zero = false;
> + tmp_page->all_zero = false;
> len = qemu_get_be32(f);
> if (len < 0 || len > compressBound(TARGET_PAGE_SIZE)) {
> error_report("Invalid compressed data length: %d", len);
> @@ -3790,16 +3789,14 @@ static int ram_load_postcopy(QEMUFile *f)
> }
>
> if (!ret && place_needed) {
> - if (all_zero) {
> - ret = postcopy_place_page_zero(mis, host_page, block);
> + if (tmp_page->all_zero) {
> + ret = postcopy_place_page_zero(mis, tmp_page->host_addr,
> block);
> } else {
> - ret = postcopy_place_page(mis, host_page, place_source,
> + ret = postcopy_place_page(mis, tmp_page->host_addr,
> place_source,
> block);
> }
> place_needed = false;
> - target_pages = 0;
> - /* Assume we have a zero page until we detect something
> different */
> - all_zero = true;
> + postcopy_temp_page_reset(tmp_page);
> }
> }
>
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 41e3238798..0ccd7e5e3f 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2579,6 +2579,18 @@ void qemu_loadvm_state_cleanup(void)
> /* Return true if we should continue the migration, or false. */
> static bool postcopy_pause_incoming(MigrationIncomingState *mis)
> {
> + int i;
> +
> + /*
> + * If network is interrupted, any temp page we received will be useless
> + * because we didn't mark them as "received" in receivedmap. After a
> + * proper recovery later (which will sync src dirty bitmap with
> receivedmap
> + * on dest) these cached small pages will be resent again.
> + */
> + for (i = 0; i < mis->postcopy_channels; i++) {
> + postcopy_temp_page_reset(&mis->postcopy_tmp_pages[i]);
> + }
> +
> trace_postcopy_pause_incoming();
>
> assert(migrate_postcopy_ram());
> --
> 2.32.0
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
