Re: [PATCH 1/3] migration/postcopy: not necessary to do discard when canonicalizing bitmap

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

* Wei Yang (richardw.yang@linux.intel.com) wrote:
> All pages, either partially sent or partially dirty, will be discarded in
> postcopy_send_discard_bm_ram(), since we update the unsentmap to be
> unsentmap = unsentmap | dirty in ram_postcopy_send_discard_bitmap().
> 
> This is not necessary to do discard when canonicalizing bitmap. And by
> doing so, we separate the page discard into two individual steps:
> 
>   * canonicalize bitmap
>   * discard page
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Yes, I think when I originally wrote it, the set of pages that was
discarded was different; I think it was actually the set of 
!unsent & dirty - i.e. only pages that had been sent and then redirtied;
it later got reworked to include unsent pages as well - so this lot can
be simplified.



Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/ram.c | 14 +-------------
>  1 file changed, 1 insertion(+), 13 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 35552c090b..075ddc468c 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2928,7 +2928,7 @@ static int postcopy_each_ram_send_discard(MigrationState *ms)
>  }
>  
>  /**
> - * postcopy_chunk_hostpages_pass: canocalize bitmap in hostpages
> + * postcopy_chunk_hostpages_pass: canonicalize bitmap in hostpages
>   *
>   * Helper for postcopy_chunk_hostpages; it's called twice to
>   * canonicalize the two bitmaps, that are similar, but one is
> @@ -2991,18 +2991,6 @@ static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>                                                               host_ratio);
>              run_start = QEMU_ALIGN_UP(run_start, host_ratio);
>  
> -            /* Tell the destination to discard this page */
> -            if (unsent_pass || !test_bit(fixup_start_addr, unsentmap)) {
> -                /* For the unsent_pass we:
> -                 *     discard partially sent pages
> -                 * For the !unsent_pass (dirty) we:
> -                 *     discard partially dirty pages that were sent
> -                 *     (any partially sent pages were already discarded
> -                 *     by the previous unsent_pass)
> -                 */
> -                postcopy_discard_send_range(ms, fixup_start_addr, host_ratio);
> -            }
> -
>              /* Clean up the bitmap */
>              for (page = fixup_start_addr;
>                   page < fixup_start_addr + host_ratio; page++) {
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK