Re: [Qemu-devel] [PATCH v9 4/8] migration: API to clear bits of guest free pages from the dirty bitmap

[Peter Xu]

On Thu, Nov 15, 2018 at 06:08:00PM +0800, Wei Wang wrote:
> This patch adds an API to clear bits corresponding to guest free pages
> from the dirty bitmap. Spilt the free page block if it crosses the QEMU
> RAMBlock boundary.
> 
> Signed-off-by: Wei Wang <address@hidden>
> CC: Dr. David Alan Gilbert <address@hidden>
> CC: Juan Quintela <address@hidden>
> CC: Michael S. Tsirkin <address@hidden>
> CC: Peter Xu <address@hidden>
> ---
>  include/migration/misc.h |  2 ++
>  migration/ram.c          | 48 
> ++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 50 insertions(+)
> 
> diff --git a/include/migration/misc.h b/include/migration/misc.h
> index 4ebf24c..113320e 100644
> --- a/include/migration/misc.h
> +++ b/include/migration/misc.h
> @@ -14,11 +14,13 @@
>  #ifndef MIGRATION_MISC_H
>  #define MIGRATION_MISC_H
>  
> +#include "exec/cpu-common.h"
>  #include "qemu/notify.h"
>  
>  /* migration/ram.c */
>  
>  void ram_mig_init(void);
> +void qemu_guest_free_page_hint(void *addr, size_t len);
>  
>  /* migration/block.c */
>  
> diff --git a/migration/ram.c b/migration/ram.c
> index ef69dbe..229b791 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3131,6 +3131,54 @@ static void ram_state_resume_prepare(RAMState *rs, 
> QEMUFile *out)
>  }
>  
>  /*
> + * This function clears bits of the free pages reported by the caller from 
> the
> + * migration dirty bitmap. @addr is the host address corresponding to the
> + * start of the continuous guest free pages, and @len is the total bytes of
> + * those pages.
> + */
> +void qemu_guest_free_page_hint(void *addr, size_t len)
> +{
> +    RAMBlock *block;
> +    ram_addr_t offset;
> +    size_t used_len, start, npages;
> +    MigrationState *s = migrate_get_current();
> +
> +    /* This function is currently expected to be used during live migration 
> */
> +    if (!migration_is_setup_or_active(s->state)) {
> +        return;
> +    }
> +
> +    for (; len > 0; len -= used_len) {
> +        block = qemu_ram_block_from_host(addr, false, &offset);
> +        assert(block);
> +
> +        /*
> +         * This handles the case that the RAMBlock is resized after the free
> +         * page hint is reported.
> +         */
> +        if (unlikely(offset > block->used_length)) {
> +            return;
> +        }

Again, is it possible to resize during migration?

So I think the check is fine, but uncertain about the comment.

And shall we print something if that happened?  We can use
error_report_once(), and squashing the above assert:

  if (!block || offset > block->used_length) {
    /* should never happen, but if it happens we ignore the hints and warn */
    error_report_once("...");
    return;
  }

What do you think?

> +
> +        if (len <= block->used_length - offset) {
> +            used_len = len;
> +        } else {
> +            used_len = block->used_length - offset;
> +            addr += used_len;

Maybe moving this line into the for() could be a bit better?

  for (; len > 0; len -= used_len, addr += used_len) {

> +        }
> +
> +        start = offset >> TARGET_PAGE_BITS;
> +        npages = used_len >> TARGET_PAGE_BITS;
> +
> +        qemu_mutex_lock(&ram_state->bitmap_mutex);
> +        ram_state->migration_dirty_pages -=
> +                      bitmap_count_one_with_offset(block->bmap, start, 
> npages);
> +        bitmap_clear(block->bmap, start, npages);
> +        qemu_mutex_unlock(&ram_state->bitmap_mutex);
> +    }
> +}
> +
> +/*
>   * Each of ram_save_setup, ram_save_iterate and ram_save_complete has
>   * long-running RCU critical section.  When rcu-reclaims in the code
>   * start to become numerous it will be necessary to reduce the
> -- 
> 1.8.3.1
> 

Regards,

-- 
Peter Xu