Re: [PATCH v3 3/7] support UFFD write fault processing in ram_save_iterate()

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> In this particular implementation the same single migration
> thread is responsible for both normal linear dirty page
> migration and procesing UFFD page fault events.
> 
> Processing write faults includes reading UFFD file descriptor,
> finding respective RAM block and saving faulting page to
> the migration stream. After page has been saved, write protection
> can be removed. Since asynchronous version of qemu_put_buffer()
> is expected to be used to save pages, we also have to flush
> migraion stream prior to un-protecting saved memory range.
> 
> Write protection is being removed for any previously protected
> memory chunk that has hit the migration stream. That's valid
> for pages from linear page scan along with write fault pages.
> 
> Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
> ---
>  migration/ram.c | 124 ++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 115 insertions(+), 9 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 7f273c9996..08a1d7a252 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -314,6 +314,8 @@ struct RAMState {
>      ram_addr_t last_page;
>      /* last ram version we have seen */
>      uint32_t last_version;
> +    /* 'write-tracking' migration is enabled */
> +    bool ram_wt_enabled;
>      /* We are in the first round */
>      bool ram_bulk_stage;
>      /* The free page optimization is enabled */
> @@ -574,8 +576,6 @@ static int uffd_protect_memory(int uffd, hwaddr start, hwaddr length, bool wp)
>      return 0;
>  }
>  
> -__attribute__ ((unused))
> -static int uffd_read_events(int uffd, struct uffd_msg *msgs, int count);
>  __attribute__ ((unused))
>  static bool uffd_poll_events(int uffd, int tmo);
>  
> @@ -1929,6 +1929,86 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>      return pages;
>  }
>  
> +/**
> + * ram_find_block_by_host_address: find RAM block containing host page
> + *
> + * Returns true if RAM block is found and pss->block/page are
> + * pointing to the given host page, false in case of an error
> + *
> + * @rs: current RAM state
> + * @pss: page-search-status structure
> + */
> +static bool ram_find_block_by_host_address(RAMState *rs, PageSearchStatus *pss,
> +        hwaddr page_address)
> +{
> +    bool found = false;
> +
> +    pss->block = rs->last_seen_block;
> +    do {
> +        if (page_address >= (hwaddr) pss->block->host &&
> +            (page_address + TARGET_PAGE_SIZE) <=
> +                    ((hwaddr) pss->block->host + pss->block->used_length)) {
> +            pss->page = (unsigned long)
> +                    ((page_address - (hwaddr) pss->block->host) >> TARGET_PAGE_BITS);
> +            found = true;
> +            break;
> +        }
> +
> +        pss->block = QLIST_NEXT_RCU(pss->block, next);
> +        if (!pss->block) {
> +            /* Hit the end of the list */
> +            pss->block = QLIST_FIRST_RCU(&ram_list.blocks);
> +        }
> +    } while (pss->block != rs->last_seen_block);
> +
> +    rs->last_seen_block = pss->block;
> +    /*
> +     * Since we are in the same loop with ram_find_and_save_block(),
> +     * need to reset pss->complete_round after switching to
> +     * other block/page in pss.
> +     */
> +    pss->complete_round = false;
> +
> +    return found;
> +}
> +
> +/**
> + * get_fault_page: try to get next UFFD write fault page and, if pending fault
> + *   is found, put info about RAM block and block page into pss structure
> + *
> + * Returns true in case of UFFD write fault detected, false otherwise
> + *
> + * @rs: current RAM state
> + * @pss: page-search-status structure
> + *
> + */
> +static bool get_fault_page(RAMState *rs, PageSearchStatus *pss)
> +{
> +    struct uffd_msg uffd_msg;
> +    hwaddr page_address;
> +    int res;
> +
> +    if (!rs->ram_wt_enabled) {
> +        return false;
> +    }
> +
> +    res = uffd_read_events(rs->uffdio_fd, &uffd_msg, 1);
> +    if (res <= 0) {
> +        return false;
> +    }
> +
> +    page_address = uffd_msg.arg.pagefault.address;
> +    if (!ram_find_block_by_host_address(rs, pss, page_address)) {
> +        /* In case we couldn't find respective block, just unprotect faulting page */
> +        uffd_protect_memory(rs->uffdio_fd, page_address, TARGET_PAGE_SIZE, false);
> +        error_report("ram_find_block_by_host_address() failed: address=0x%0lx",
> +                     page_address);
> +        return false;
> +    }
> +
> +    return true;
> +}
> +
>  /**
>   * ram_find_and_save_block: finds a dirty page and sends it to f
>   *
> @@ -1955,25 +2035,50 @@ static int ram_find_and_save_block(RAMState *rs, bool last_stage)
>          return pages;
>      }
>  
> +    if (!rs->last_seen_block) {
> +        rs->last_seen_block = QLIST_FIRST_RCU(&ram_list.blocks);
> +    }
>      pss.block = rs->last_seen_block;
>      pss.page = rs->last_page;
>      pss.complete_round = false;
>  
> -    if (!pss.block) {
> -        pss.block = QLIST_FIRST_RCU(&ram_list.blocks);
> -    }
> -
>      do {
> +        ram_addr_t page;
> +        ram_addr_t page_to;
> +
>          again = true;
> -        found = get_queued_page(rs, &pss);
> -
> +        /* In case of 'write-tracking' migration we first try
> +         * to poll UFFD and get write page fault event */
> +        found = get_fault_page(rs, &pss);
> +        if (!found) {
> +            /* No trying to fetch something from the priority queue */
> +            found = get_queued_page(rs, &pss);
> +        }
>          if (!found) {
>              /* priority queue empty, so just search for something dirty */
>              found = find_dirty_block(rs, &pss, &again);
>          }
>  
>          if (found) {
> +            page = pss.page;
>              pages = ram_save_host_page(rs, &pss, last_stage);
> +            page_to = pss.page;
> +
> +            /* Check if page is from UFFD-managed region */
> +            if (pss.block->flags & RAM_UF_WRITEPROTECT) {
> +                hwaddr page_address = (hwaddr) pss.block->host +
> +                        ((hwaddr) page << TARGET_PAGE_BITS);
> +                hwaddr run_length = (hwaddr) (page_to - page + 1) << TARGET_PAGE_BITS;
> +                int res;
> +
> +                /* Flush async buffers before un-protect */
> +                qemu_fflush(rs->f);
> +                /* Un-protect memory range */
> +                res = uffd_protect_memory(rs->uffdio_fd, page_address, run_length, false);
> +                if (res < 0) {
> +                    break;
> +                }
> +            }

Please separate that out into a separate function.

Dave

>          }
>      } while (!pages && again);
>  
> @@ -2086,7 +2191,8 @@ static void ram_state_reset(RAMState *rs)
>      rs->last_sent_block = NULL;
>      rs->last_page = 0;
>      rs->last_version = ram_list.version;
> -    rs->ram_bulk_stage = true;
> +    rs->ram_wt_enabled = migrate_track_writes_ram();
> +    rs->ram_bulk_stage = !rs->ram_wt_enabled;
>      rs->fpo_enabled = false;
>  }
>  
> -- 
> 2.25.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK