On Thu, Feb 03, 2022 at 03:08:39PM +0000, Dr. David Alan Gilbert wrote: > * 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). > > Generally OK, some minor nits. > > > Signed-off-by: Peter Xu <pet...@redhat.com> > > --- > > migration/migration.h | 35 +++++++++++++++++++++++++++- > > migration/postcopy-ram.c | 50 +++++++++++++++++++++++++++++----------- > > migration/ram.c | 43 +++++++++++++++++----------------- > > 3 files changed, 91 insertions(+), 37 deletions(-) > > > > diff --git a/migration/migration.h b/migration/migration.h > > index 8130b703eb..8bb2931312 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) */ > > + int target_pages; > > Can we take the opportunity to convert this to an unsigned?
Sure. > > > + /* 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. > > + */ > > + int postcopy_channels; > > Also unsigned? OK. > > > + /* > > + * 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; > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c > > index e662dd05cc..d78e1b9373 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(sizeof(PostcopyTmpPage) * > > channels); > > I noticed we've started using g_malloc0_n in a few places Sure. > > > + 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)); > > Please call postcopy_temp_pages_cleanup here to cleanup previous pages > that were succesfully allocated. It'll be cleaned up later here: loadvm_postcopy_handle_listen postcopy_ram_incoming_setup postcopy_temp_pages_setup postcopy_ram_incoming_cleanup <---------- if fail above, go here postcopy_temp_pages_cleanup Thanks, -- Peter Xu