Re: [Qemu-devel] [PATCH v4 2/4] migration: API to clear bits of guest free pages from the dirty bitmap

[Dr. David Alan Gilbert]

* Wei Wang (address@hidden) 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>
> ---
>  include/migration/misc.h |  2 ++
>  migration/ram.c          | 21 +++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/include/migration/misc.h b/include/migration/misc.h
> index 77fd4f5..fae1acf 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 5e33e5c..e172798 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2189,6 +2189,27 @@ static int ram_init_all(RAMState **rsp)
>      return 0;
>  }
>  

This could do with some comments

> +void qemu_guest_free_page_hint(void *addr, size_t len)
> +{
> +    RAMBlock *block;
> +    ram_addr_t offset;
> +    size_t used_len, start, npages;

From your use I think the addr and len are coming raw from the guest;
so we need to take some care.

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

That initialisation of used_len is unusual; I'd rather put that
in the body.

> +        block = qemu_ram_block_from_host(addr, false, &offset);

CHeck for block != 0

> +        if (unlikely(offset + len > block->used_length)) {

I think to make that overflow safe, that should be:
  if (len > (block->used_length - offset)) {

But we'll need another test before it, because qemu_ram_block_from_host
seems to check max_length not used_length, so we need to check
for offset > block->used_length first

> +            used_len = block->used_length - offset;
> +            addr += used_len;
> +        }
> +
> +        start = offset >> TARGET_PAGE_BITS;
> +        npages = used_len >> TARGET_PAGE_BITS;
> +        ram_state->migration_dirty_pages -=
> +                      bitmap_count_one_with_offset(block->bmap, start, 
> npages);
> +        bitmap_clear(block->bmap, start, npages);

If this is happening while the migration is running, this isn't safe -
the migration code could clear a bit at about the same point this
happens, so that the count returned by bitmap_count_one_with_offset
wouldn't match the word that was cleared by bitmap_clear.

The only way I can see to fix it is to run over the range using
bitmap_test_and_clear_atomic, using the return value to decrement
the number of dirty pages.
But you also need to be careful with the update of the
migration_dirty_pages value itself, because that's also being read
by the migration thread.

Dave

> +    }
> +}
> +
>  /*
>   * Each of ram_save_setup, ram_save_iterate and ram_save_complete has
>   * long-running RCU critical section.  When rcu-reclaims in the code
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK