Re: [Qemu-devel] [PATCH v16 09/22] qcow2: autoloading dirty bitmaps

[Max Reitz <mreitz@redhat.com>]

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On 25.02.2017 18:07, Vladimir Sementsov-Ogievskiy wrote:
> Auto loading bitmaps are bitmaps in Qcow2, with the AUTO flag set. They
> are loaded when the image is opened and become BdrvDirtyBitmaps for the
> corresponding drive.
> 
> Extra data in bitmaps is not supported for now.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/qcow2-bitmap.c | 367 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  block/qcow2.c        |   7 +
>  block/qcow2.h        |   2 +
>  3 files changed, 376 insertions(+)
> 
> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
> index b8e472b3e8..73a6e87038 100644
> --- a/block/qcow2-bitmap.c
> +++ b/block/qcow2-bitmap.c

[...]

> +static int update_ext_header_and_dir_in_place(BlockDriverState *bs,
> +                                              Qcow2BitmapList *bm_list)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    int ret;
> +
> +    if (!(s->autoclear_features & QCOW2_AUTOCLEAR_BITMAPS) ||
> +        bm_list == NULL || QSIMPLEQ_EMPTY(bm_list) ||
> +        bitmap_list_count(bm_list) != s->nb_bitmaps)
> +    {
> +        return -EINVAL;
> +    }
> +
> +    s->autoclear_features &= ~(uint64_t)QCOW2_AUTOCLEAR_BITMAPS;
> +    ret = update_header_sync(bs);
> +    if (ret < 0) {
> +        /* Two variants are possible here:
> +         * 1. Autoclear flag is dropped, all bitmaps will be lost.
> +         * 2. Autoclear flag is not dropped, old state is left.
> +         */
> +        return ret;
> +    }
> +
> +    /* autoclear bit is not set, so we can safely update bitmap directory */
> +
> +    ret = bitmap_list_store(bs, bm_list, &s->bitmap_directory_offset,
> +                            &s->bitmap_directory_size, true);
> +    if (ret < 0) {
> +        /* autoclear bit is cleared, so all leaked clusters would be removed on
> +         * qemu-img check */
> +        return ret;
> +    }
> +
> +    ret = update_header_sync(bs);
> +    if (ret < 0) {
> +        /* autoclear bit is cleared, so all leaked clusters would be removed on
> +         * qemu-img check */
> +        return ret;
> +    }
> +
> +    s->autoclear_features |= QCOW2_AUTOCLEAR_BITMAPS;
> +    return update_header_sync(bs);
> +    /* If final update_header_sync() fails, two variants are possible:
> +     * 1. Autoclear flag is not set, all bitmaps will be lost.
> +     * 2. Autoclear flag is set, header and directory are successfully updated.
> +     */

Oh, nice. Thanks for the explanations, too. This way I'm very much fine
with update_header_sync() failing if bdrv_flush() fails.

> +}
> +
> +/* for g_slist_foreach for GSList of BdrvDirtyBitmap* elements */
> +static void release_dirty_bitmap_helper(gpointer bitmap,
> +                                        gpointer bs)
> +{
> +    bdrv_release_dirty_bitmap(bs, bitmap);
> +}

[...]

> diff --git a/block/qcow2.c b/block/qcow2.c
> index 6cd411f9c5..f4bb9d46de 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1237,6 +1237,13 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
>          goto fail;
>      }
>  
> +    qcow2_load_autoloading_dirty_bitmaps(bs, &local_err);
> +    if (local_err != NULL) {
> +        error_propagate(errp, local_err);
> +        ret = -EINVAL;
> +        goto fail;
> +    }
> +
>      /* Clear unknown autoclear feature bits */
>      need_update_header |= s->autoclear_features & ~QCOW2_AUTOCLEAR_MASK;
>  

I think it would be better to load the dirty bitmaps after clearing
unknown autoclear bits, because loading the bitmaps may result in
modifications of the image file.

Max


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