Re: [Qemu-devel] [PATCH RFC v2 4/8] block: add dirty-dirty bitmaps

[John Snow <jsnow@redhat.com>]

I had hoped it wouldn't come to this :)

On 01/27/2015 05:56 AM, Vladimir Sementsov-Ogievskiy wrote:
> A dirty-dirty bitmap is a dirty bitmap for BdrvDirtyBitmap. It tracks
> set/unset changes of BdrvDirtyBitmap.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@parallels.com>
> ---
>   block.c               | 44 ++++++++++++++++++++++++++++++++++++++++++++
>   include/block/block.h |  5 +++++
>   2 files changed, 49 insertions(+)
>
> diff --git a/block.c b/block.c
> index 9860fc1..8ab724b 100644
> --- a/block.c
> +++ b/block.c
> @@ -53,6 +53,7 @@
>
>   struct BdrvDirtyBitmap {
>       HBitmap *bitmap;
> +    HBitmap *dirty_dirty_bitmap;

Just opinions: Maybe we can call it the "meta_bitmap" to help keep the 
name less confusing, and accompany it with a good structure comment here 
to explain what the heck it's for.

If you feel that's a good idea; s/dirty_dirty_/meta_/g below.

Regardless, let's make sure this patch adds documentation for it.

>       BdrvDirtyBitmap *originator;
>       int64_t size;
>       int64_t granularity;
> @@ -5373,6 +5374,30 @@ BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
>       return originator;
>   }
>
> +HBitmap *bdrv_create_dirty_dirty_bitmap(BdrvDirtyBitmap *bitmap,
> +                                        uint64_t granularity)
> +{
> +    uint64_t sector_granularity;
> +
> +    assert((granularity & (granularity - 1)) == 0);
> +
> +    granularity *= 8 * bitmap->granularity;
> +    sector_granularity = granularity >> BDRV_SECTOR_BITS;
> +    assert(sector_granularity);
> +
> +    bitmap->dirty_dirty_bitmap =
> +        hbitmap_alloc(bitmap->size, ffsll(sector_granularity) - 1);
> +
> +    return bitmap->dirty_dirty_bitmap;
> +}
> +
> +void bdrv_release_dirty_dirty_bitmap(BdrvDirtyBitmap *bitmap)
> +{
> +    if (bitmap->dirty_dirty_bitmap) {
> +        hbitmap_free(bitmap->dirty_dirty_bitmap);
> +        bitmap->dirty_dirty_bitmap = NULL;
> +    }
> +}
>
>   void bdrv_print_dirty_bitmap(BdrvDirtyBitmap *bitmap)
>   {
> @@ -5447,6 +5472,9 @@ void bdrv_release_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
>           if (bm == bitmap) {
>               QLIST_REMOVE(bitmap, list);
>               hbitmap_free(bitmap->bitmap);
> +            if (bitmap->dirty_dirty_bitmap) {
> +                hbitmap_free(bitmap->dirty_dirty_bitmap);
> +            }
>               g_free(bitmap->name);
>               g_free(bitmap);
>               return;
> @@ -5534,6 +5562,10 @@ void bdrv_set_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap,
>   {
>       if (bitmap->enabled) {
>           hbitmap_set(bitmap->bitmap, cur_sector, nr_sectors);
> +
> +        if (bitmap->dirty_dirty_bitmap) {
> +            hbitmap_set(bitmap->dirty_dirty_bitmap, cur_sector, nr_sectors);
> +        }
>       }
>   }
>
> @@ -5541,6 +5573,9 @@ void bdrv_reset_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap,
>                                int64_t cur_sector, int nr_sectors)
>   {
>       hbitmap_reset(bitmap->bitmap, cur_sector, nr_sectors);
> +    if (bitmap->dirty_dirty_bitmap) {
> +        hbitmap_set(bitmap->dirty_dirty_bitmap, cur_sector, nr_sectors);
> +    }
>   }
>
>   /**
> @@ -5550,6 +5585,9 @@ void bdrv_reset_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap,
>   void bdrv_clear_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
>   {
>       hbitmap_reset(bitmap->bitmap, 0, bitmap->size);
> +    if (bitmap->dirty_dirty_bitmap) {
> +        hbitmap_set(bitmap->dirty_dirty_bitmap, 0, bitmap->size);
> +    }
>   }
>
>   const char *bdrv_dirty_bitmap_name(const BdrvDirtyBitmap *bitmap)
> @@ -5597,6 +5635,9 @@ static void bdrv_set_dirty(BlockDriverState *bs, int64_t cur_sector,
>               continue;
>           }
>           hbitmap_set(bitmap->bitmap, cur_sector, nr_sectors);
> +        if (bitmap->dirty_dirty_bitmap) {
> +            hbitmap_set(bitmap->dirty_dirty_bitmap, cur_sector, nr_sectors);
> +        }
>       }
>   }
>
> @@ -5606,6 +5647,9 @@ static void bdrv_reset_dirty(BlockDriverState *bs, int64_t cur_sector,
>       BdrvDirtyBitmap *bitmap;
>       QLIST_FOREACH(bitmap, &bs->dirty_bitmaps, list) {
>           hbitmap_reset(bitmap->bitmap, cur_sector, nr_sectors);
> +        if (bitmap->dirty_dirty_bitmap) {
> +            hbitmap_set(bitmap->dirty_dirty_bitmap, cur_sector, nr_sectors);
> +        }
>       }
>   }
>
> diff --git a/include/block/block.h b/include/block/block.h
> index 0890cd2..648b0a9 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -4,6 +4,7 @@
>   #include "block/aio.h"
>   #include "qemu-common.h"
>   #include "qemu/option.h"
> +#include "qemu/hbitmap.h"
>   #include "block/coroutine.h"
>   #include "block/accounting.h"
>   #include "qapi/qmp/qobject.h"
> @@ -473,6 +474,10 @@ void bdrv_dirty_bitmap_deserialize_part0(BdrvDirtyBitmap *bitmap,
>                                            uint64_t start, uint64_t count);
>   void bdrv_dirty_bitmap_deserialize_finish(BdrvDirtyBitmap *bitmap);
>
> +HBitmap *bdrv_create_dirty_dirty_bitmap(BdrvDirtyBitmap *bitmap,
> +                                        uint64_t granularity);
> +void bdrv_release_dirty_dirty_bitmap(BdrvDirtyBitmap *bitmap);
> +
>   void bdrv_enable_copy_on_read(BlockDriverState *bs);
>   void bdrv_disable_copy_on_read(BlockDriverState *bs);
>
>

Looks correct, it just needs documentation in my opinion to help explain 
what this extra bitmap is for.

Musings / opinions:

I also think that at this point, we may want to make bdrv_reset_dirty 
and bdrv_set_dirty call the bdrv_reset_dirty_bitmap and 
bdrv_set_dirty_bitmap functions instead of re-implementing the behavior.

I used to think it was fine as-is, but the more conditions we add to how 
these bitmaps should be accessed, the more I think limiting the 
interface to as few functions as possible is a good idea.

Maybe I'll even do that myself. It might even be nice to split off all 
the bitmap functions off into something like block/dirty_bitmaps.c as 
the complexity creeps up and we're trying to improve the maintainability 
of block.c.

Anyway, we can worry about that in a later series.